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Full text of "Physiological aspects of maize (Zea mays L.) yield"

PHYSIOLOGICAL ASPECTS OF MAIZE (Zea mays L.) YIELD 



By 

RAUL RENE VALLE MELENDEZ 



A DISSERTATION PRESENTED TO THE GRADUATE COUNCIL OF 

THE UNIVERSITY OF FLORIDA IN PARTIAL 

FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF 

DOCTOR OF PHILOSOPHY 



UNIVERSITY OF FLORIDA 
1981 



ACKNOWLEDGMENTS 

I wish to express my sincere appreciation to Dr. D. E. McCloud 
for being chairman of my supervisory committee during most of my 
program; his guidance and assistance were invaluable throughout my 
graduate work. I also would like to thank Dr. K. J. Boote, Dr. W. G. 
Duncan, Dr. E. S. Horner, and Dr. W. G. Blue for being members of my 
committee and for their support during my graduate studies. 

Thanks are also in order to Dr. F. P. Gardner for assuming respon- 
sibility as chairman of my supervisory committee and for helping me 
edit this dissertation and to Dr. E. G. Rodgers for his help in getting 
me started into Graduate School. 

Words of gratitude are given to the Ministry of Natural Resources 
in Honduras for the permission to continue my graduate work and to the 
Organization of American States for financial support without which my 
work would hardly have been accomplished. 

Special words of thanks go to all graduate students of the physi- 
ology group; to Mr. R. A. Hill for his help in the conduction of the 
field experiments; to Mrs. Carolyn Meyer for typing this manuscript; 
to Mrs. Beth Chandler for drawing the figures; and also to all those 
persons who directly or indirectly helped me in this endeavor. 

I gratefully acknowledge my wife, Ivete, for her understanding, 
patience, and encouragement. Thanks and excuses are presented to Raul 
Rene and Marco Antonio, children sometimes neglected. 

11 



I am grateful for the moral support of my mother, family, and 
friends throughout our stay in Gainesville. I want to recognize the 
great value of the teachings of my late father, to whom education was 
a primary sign of progress. 

To God I humbly give thanks. 



m 



TABLE OF CONTENTS 

Page 

ACKNOWLEDGMENTS i i 

LIST OF TABLES vi 

LIST OF FIGURES vii 

ABSTRACT ix 

INTRODUCTION 1 

LITERATURE REVIEW 3 

MATERIALS AND METHODS 19 

RESULTS AND DISCUSSION 28 

1978 Experiment 28 

Vegetative Yields 28 

Crop Growth and Ear Growth Rates 30 

Brix Readings 31 

1979 Experiment 31 

Cultivar Characteristics 31 

Total Dry Weight 34 

Root Dry Weight 37 

Stal k Dry Weight 37 

Leaf Dry Weight 39 

Ear Dry Weight 41 

Dry Matter Distribution 41 

Crop Growth Rates 44 

Ear Growth Rates 50 

Ear Effective Filling Periods 51 

Kernel Growth Components 53 

Total Available Carbohydrates 56 

Bri x Readi ngs 64 

Partitioning Coefficient 70 

Yield Dynamics 73 

SUMMARY AND CONCLUSIONS 76 

LITERATURE CITED 78 

iv 



APPENDICES Page 

A AMYLOGLUCOSIDASE-INVERTASE PROCEDURE FOR HYDRO! Y7TNfi Tnifli 
AVAILABLE CARBOHYDRATES (STARCH, SUCROSE) TO REDUaNG'' 

87 

^ ™^^- ^°^'- f'ERTILITY ANALYSIS, DRY WEIGHT OF PLANT 
COMPONENTS FOR THE 1978 AND 1979 GROWING SEASONS TOTAL 
AVAILABLE CARBOHYDRATES, AND PERCENT BRIX READINGS. !. 90 

BIOGRAPHICAL SKETCH.... 

97 



LIST OF TABLES 
Table Page 

1 Fertilizer and pesticides used in the maize 

growth analysis experiments of 1978 and 1979 22 

2 Mean Brix readings per internode and per plot for 
the hybrid corn Pioneer Brand 3369A and the inbred 

Iowa B37 studied in 1978 32 

3 Characteristics of the cultivars studied in 1979 33 

4 Dry matter distribution in percent of the total 

dry weight 43 

5 Crop growth rates, EGR, and distribution index for 

the maize cultivars grown in 1979 46 

6 Estimation of glucose required for the vegetative, 

seed, and cob components of a maize plant 48 

7 Summary of related yield parameters for the maize 
cultivars grown in 1979 52 

8 Percentages of total available carbohydrates (TAG) 
for the stalks, leaves, cobs, and grain of the 

maize cultivars studied in 1979 57 

Appendix B Tables 

B-1 The pH, nitrogen, and double-acid extractable 

nutrients in the soil used in the 1979 experiment 90 

B-2 Dry weight of plant components and LAI for the 
hybrid corn Pioneer Brand 3369A and the inbred 
line Iowa B37 investigated in 1978 91 

B-3 Dry weight of plant components for the maize culti- 
vars investigated in 1979 92 

B-4 Total available carbohydrates (TAC) in the different 

plant components of the maize cultivars grown in 1979... 93 

B-5 Average percent Brix readings per plot and for the 

maize cultivers grown in 1979 94 

B-6 Average percentage Brix readings per internode for the 

maize cultivars studied in 1979 95 

vi 



LIST OF FIGURES 
Figures Page 

1 Climatological data for the 1978 experimental 
period. Averages of 10-day periods coinciding 

wi th harvest dates 20 

2 Climatological data for the 1979 experimental 
period. Averages of 10-day periods coinciding 

wi th harvest dates 21 

3 Total, stalk, leaves, roots, and ear dry weight 
accumulation for the hybrid (H) Pioneer Brand 
3369A and the inbred line (I) Iowa B37 studied 

in 1978 29 

4 Leaf area index for the maize cultivars grown in 
1979. CH + Chapalote; CO = Coker 77; MC = Maiz 
Criollo; NT = Nal-Tel. Arrows indicate anthesis 

dates 35 

5 Total dry matter accumulation for the maize culti- 
vars grown in 1979. CH = Chapalote; CO = Coker 77; 
MC = Maiz Criollo; NT = Nal-Tel. Arrows indicate 
anthesis dates 36 

6 Stalk dry matter accumulation for the maize culti- 
vars grown in 1979. CH = Chapalote; CO = Coker 77; 
MC = Maiz Criollo; NT = Nal-Tel. Arrows indicate 
anthesi s dates 38 

7 Leaf dry matter accumulation for the maize cultivars 
grown in 1979. CH = Chapalote; CO = Coker 77; MC = 
Maiz Criollo; NT = Nal-Tel. Arrows indicate 

anthesis dates 40 

8 Ear dry matter accumulation for the maize cultivars 
grown in 1979. CH = Chapalote; CO = Coker 77; 

MC = Maiz Criollo; NT = Nal-Tel 42 

9 Kernel dry matter accumulation for the maize culti- 
vars grown in 1979. CH = Chapalote; CO = Coker 77; 

Mc = Maiz Criollo; NT = Nal-Tel 54 

vii 



Figures Page 

10 Stalk TAG weight in the maize cultivars grown in 
1979. CH = Chapalote; CO = Coker 77; MC = 
Maiz Criollo; NT = Nal-Tel. Arrows indicate 
anthesi s dates 59 

n Leaf TAG weight in the maize cultivars grown in 
1979. CH = Chapalote; GO = Coker 77; MC = Maiz 
Criollo; NT = Nal-Tel. Arrows indicate anthesis 
dates 60 

12 Gob TAG weight in the maize cultivars grown in 1979. 
CH = Chapalote; CO = Coker 77; MG = Maiz Criollo; 

NT = Nal-Tel..... 61 

13 Grain TAG weight in the maize cultivars grown in 
the 1979 growing season. CH = Chapalote; CO = 

Coker 77; MC = Maiz Criollo; NT = Nal-Tel 63 

14 Average percent Brix readings per plot in the 
cultivars grown in 1979. CH = Chapalote; CO = 

Coker 77; MC = Maiz Criollo; NT = Nal-Tel 65 

15 Average percent Brix readings per internode in 
Coker 77. Smaller numbers indicate lower inter- 
nodes. Arrow marks anthesis date 66 

16 Average percent Brix readings per internode in 
Nal-Tel. Smaller numbers indicate lower inter- 
nodes. Arrow marks anthesis date 67 

17 Average percent Brix readings per internode in 
Chapalote. Smaller numbers indicate lower inter- 
nodes. Arrow marks anthesis date 68 

18 Average percent Brix readings per internode in 
Maiz Criollo. Smaller numbers indicate lower 
internodes. Arrow marks anthesis date 69 



vm 



Abstract of Dissertation Presented to the Graduate Council 

of the University of Florida in Partial Fulfillment of the 

Requirements for the Degree of Doctor of Philosophy 

PHYSIOLOGICAL ASPECTS OF MAIZE ( Zea mays L.) YIELD 

By 

RAUL RENE VALLE MELENDEZ 

March 1981 

Chairman: F. P. Gardner 
Major Department: Agronomy 

Two experiments were conducted to study physiological reasons 
for yield differences among diverse maize ( Zea mays L. ) cultivars. 
Growth analysis techniques were used to test the hypothesis that 
high yields of cormiercial maize hybrids are due to: (1) high rates 
of dry matter accumulation, (2) a proportionally larger distribution 
of assimilates to reproductive growth, and/or (3) longer filling 
period. Attention was also given to storage of assimilates. 

In 1978 the single-cross hybrid Pioneer Brand '3369A' was com- 
pared with the inbred line Iowa B37, and in 1979 the hybrid 'Coker 77' 
was compared with two ancient Mexican races, 'Chapalote' and 'Nal-Tel', 
and a Cuban accession, 'Maiz Criollo'. These experiments were planted 
at the University of Florida Agronomy Farm in Gainesville at population 
densities expected to give approximately equal leaf area indices (LAI). 
Refractometric readings per internode and per unit area were determined 
in both experiments. Also, in 1979 total available carbohydrate (TAC) 

ix 



in plant components (stalk, leaf, cob, and grain) were measured. 
Rates of dry matter accumulation (crop growth rates), computed during 
the vegetative (CGRv) and reproductive (CGRr) phases, were examined as 
estimates of canopy capacity to produce assimilates. Ear growth rates 
(EGR) were also determined. The partitioning coefficient (PC) was used 
as an estimate of assimilate distributed to ear growth as opposed to 
vegetative growth, and effective ear filling period (EEFP) and 
effective seed filling period (ESFP) were estimates of filling periods. 

In 1978, the hybrid and inbred ear yields were drastically reduced 
by poor environmental conditions; however, the hybrid yield was signif- 
icantly greater than that of the inbred. Stalk refractometric readings 
per unit land area were higher in the inbred than in the hybrid. This 
suggested a higher accumulation of soluble solids in the inbred line, 
probably because of its lower sink capacity. 

Except for Nal-Tel, the cultivars compared in 1979 did not signif- 
icantly differ in CGRv and LAI. However, Coker 77 maintained its high 
LAI for a longer period than the other cultivars. During reproductive 
growth, all cultivars showed a decrease in canopy assimilate production. 
Although CGRr values of Chapalote and Coker 77 were not significantly 
different, the CGRr of Coker 77 was sustained for a longer period. 
Maiz Criollo and Nal-Tel had the lowest CGRr values. Partitioning 
coefficient varied among cultivars. Nal-Tel had the highest PC followed 
by Coker 77, Maiz Criollo, and Chapalote. Final ear yields for Coker 
77, Maiz Criollo, Nal-Tel and Chapalote were 1023, 776, 607, and 
578 g m-2, respectively. Total available carbohydrates were higher in 
the plant components of Coker 77 than in the other cultivars. Contri- 
butions of remobilized TAC from vegetative components to final ear 

X 



yield were estimated to be 9, 13, 21, and 26% in Coker 77, Maiz 
Criollo, Nal-Tel, and Chapalote, respectively. 

Similar CGRv values in Chapalote, Coker 77, and Maiz Criollo 
indicated similar potential to produce high yields. Chapalote, however, 
produced many barren tillers in addition to ear-bearing stalks. This 
tillering habit of Chapalote explains its lower PC and yield. Nal-Tel 
had the highest PC, but its ear yield was low probably because of its 
low LAI. Length of filling period, either as EEFP or ESFP, was not a 
significant factor in yield differences. Coker 77 had the highest 
EGR; differences in EGR accounted for most of the yield differences 
among cultivars. 

The results of these experiments suggested that under conditions 
of similar LAI, the physiological characteristics of a high-yielding 
maize cultivar are high PC, high EGR, and a longer duration of CGRr. 



XI 



INTRODUCTION 

While it is generally recognized that commercial hybrid maize 
( Zea mays L.) cultivars outyield their inbred parents, or ancient races 
(Duncan and Hesketh, 1968), the physiological and ecological bases for 
their increased yield have not yet been adequately explained, 

A basic step toward increasing the yield of any crop is to under- 
stand its pattern of dry matter accumulation. When only final economic 
yields are determined, little knowledge can be gained on how high 
yields are achieved. However, growth analysis is an effective way to 
study the dynamics of dry matter accumulation and yield physiology. 

The experiments discussed in the following pages permitted observa- 
tion and recording of physiological responses of hybrid maize when 
compared with an inbred line, and with ancient races of Central and 
North America. However, the main objective was to investigate which 
of three major hypotheses for differences in yield among cultivars best 
explained the higher yield of hybrid maize. These hypotheses are: 
i) Higher yielding cultivars have higher crop canopy photosynthetic 
efficiency. Cultivars with more efficient canopy photosynthesis would 
produce more assimilates with a given amount of solar radiation and 
should produce higher yields. The crop growth rate (CGRv) after the 
canopy has reached 97% ground cover (light interception) and prior to 
ear development, reflects canopy assimilate production which can go to 
grain fill, ii) High yielding cultivars have longer duration of the 
effective grain filling period. The longer the grain filling period a 

1 



2 

cultivar has the more solar radiation it can intercept to produce 
photosynthate for ear growth, iii) High yielding cultivars have 
different distribution of assimilates between reproductive and vegeta- 
tive growth during the period of ear establishment. A cultivar 
with greater distribution of assimilates to the reproductive sink 
during the grain setting period should have greater yield. 

It was hoped that this study would help elucidate differences in 
yield among hybrid, inbred, and ancient races in terms of physiological 
parameters. 



LITERATURE REVIEW 

Increase in dry weight is a useful definition of growth for 
scientists interested in crop productivity. Crop growth is usually 
more accurately characterized by measurement of dry weight than 
measurements of fresh weight, which can be strongly influenced by 
prevailing moisture conditions. However, dry weight increase is not a 
completely satisfactory definition of growth; because growth also 
includes germination during which dry weight is lost, cell multiplica- 
tion and increase in volume both may represent little change in dry 
weight (Salisbury and Ross, 1969). 

Dry weight increase has been described mathematically as a 
function of physiological, phenological , and environmental factors. 
Increase in dry weight with time is usually characterized by a sigmoidal 
curve (Leopold and Kriedemann, 1975), in which three primary phases are 
recognized: expansion, linear, and senescence (Richards, 1969). In 
the expansion phase, the growth rate (increase in dry weight per unit 
of time) is initially slow but the rate increases continuously as more 
dry weight is added. Growth of a higher plant during its exponential 
phase is analogous to the accumulation of capital at a continuous com- 
pound interest and can be described by the equation Wi = Wq (1 + r)t, 
where Wq is the initial weight, r is the rate of growth or capacity to 
add dry weight (Blackman, 1919), and Wi is the total dry weight after 
a certain time t. Accumulation of dry weight is exponential until 
self-shading or other conditions prevent the increasing leaf area from 

3 



■'■ 4 

producing a proportionate increase in the weight of the plant (Watson, 
1958; Leopold and Kriedemann, 1975; Duncan et al . , 1967). 

The end of the expansion phase marks the beginning of the linear 
phase in which the increase in dry matter continues at a constant 
rate. The final, senescence phase is characterized by a decrease in 
growth rate as the crop approaches maturity and begins to senesce 
(Salisbury and Ross, 1978). 

Growth analysis, by periodic harvest, is a useful tool to charac- 
terize and describe these growth phases of single plants or plant 
communities (McKinion et al., 1974). The use of relative growth rate 
(RGR, g g"^ time), net assimilation rate (NAR, g dm-^ time"M> and 
leaf area ratio (LAR, dm^ g-i) to quantitatively analyze plant growth 
has become known as "growth analysis." The measurement of the total 
dry weight of plant material per unit land area and the measurement 
of the assimilatory system are the two parameters needed to conduct 
a growth analysis of a plant community (Radford, 1967). The assimila- 
tory system of a plant community is generally computed as the total 
leaf area (one side) per unit land area and is known as the leaf 
area index (LAI) of a canopy (Watson, 1947a and 1947b). 

The introduction of the crop growth rate (CGR) function (Watson, 
1958), to the traditional "growth analysis," has been recognized 
(Williams et al., 1965b) as the most meaningful growth function, since 
it represents the net results of photosynthesis, respiration, and 
canopy area interactions. As noted by Williams et al . , CGR is also 
representative of the most conmon agronomic measurement, i.e., yield 
of dry matter per unit of land area. 



The CGR is defined as the increase in plant material per unit land 
area per unit time. The mean CGR over a time period t] to t2 is given 
by CGR = W2 - Wi / t2 - t] , where W] and W2 designate the total dry 
weights at periods t] and t2. respectively (Watson, 1958). Thus, CGR 
represents total dry matter productivity of a plant community and, 
except for a small mineral component, it can be equated as an estimation 
of net carbon fixed for a crop canopy (Duncan et al . , 1978). 

Crop growth rate shows a close relationship to LAI in all plant 
communities, especially below LAI of four (Duncan, 1975). For certain 
crops such as kale ( Brassica oleracea L.), subterranean clover 
( Tri folium subterraneum L. ) , sunflower ( Helianthus annuus L. ) , and rice 
( Oryza sativa L.) optimum LAI values have been shown to exist (Watson, 
1958; Black, 1963; Takeda, 1951; Hiroi and Monsi, 1966; Yoshida, 1972). 
According to these authors increasing LAI beyond the optimum caused a 
decline in CGR, which was attributed to mutual shading of leaves, such 
that further increase in leaf area did not compensate for the reduction 
in net photosynthesis because of less effective illumination. The 
community still gained dry weight at high LAI, but the rate diminished 
(Leopold and Kriedemann, 1975). However, experiments with mixed 
pastures by Brougham (1956), sugar beet ( Beta vulgaris L.) by Watson 
(1958), maize ( Zea mays L.) by Williams et al , (1965b), soybeans 
( Glycine max L., Merr.) by Shibles and Weber (1966), and rape ( Brassica 
napus L.) by Clarke and Simpson (1978) have not displayed an optimum 
LAI. The CGR response to LAI for corn does not indicate a peak in CGR 
at some optimum LAI (Williams et al , , 1965b; Duncan, 1975); rather, 
the trend of the curve, after the initial rise at the low end of the LAI 
range, is toward an asymptotic plateau. Decline in CGR at later stages 



of the growth cycle of corn has been associated partly to the decline 
in radiation received as the season advances (Duncan et al . , 1967; 
Williams et al . , 1968). The asymptotic relationship between CGR and 
LAI has also been demonstrated for wheat ( Triticum aestivum L. em Thell.) 
by Puckridge and Donald (1967), soybean by Shibles and Weber (1965), 
and rape by Clarke and Simpson (1978). One explanation for the plateau 
at high LAI values has been elucidated by experiments in cotton 
( Gossypium hirsutum L.) by Ludwig et al . (1965), and white clover 
( Tri folium repens L. ) by McCree and Troughton (1966a, 1966b). These 
authors demonstrated that respiration in shade-adapted leaves of the 
basal strata in dense canopies of these crops is lower than in leaves 
exposed to more intense illumination. Thus, CGR may attain a plateau 
rather than decline beyond an optimal value of LAI. Watson (1958) 
stated and Williams et al . (1965a, 1965b, 1968) confirmed that the 
form of the curve relating CGR to LAI and the maximum value of CGR 
are determined by the way in which the spatial distribution of leaves 
effectsthe utilization of incident radiation. Furthermore, at high 
LAI vertical leaves allow more uniform light incidence, enhancing CGR 
(Williams et al., 1968). This depends upon light intensity (Kriedemann 
and Smart, 1971) and adaptation in respiration rates, since lower 
leaves are not parasitic as was once thought (Ludwig et al . , 1965; 
McCree and Troughton, 1966a, 1966b). 

Computer simulations by Duncan (1971) suggested that leaf angle 
in maize has small effects on photosynthetic rates of a crop canopy 
per unit land area at LAI values lower than four. Williams et al . 
(1965a, 1965b, 1968) have shown, however, that the difference in light 
interception in the range of LAI of 2.7 to 4.5 can be as high as 30% 



J 



and that CGR varied proportionally. Their research was conducted with 
a single-cross hybrid, Dekalb Brand 805, over a wide range of population 
densities varying from 1.15 to 12.5 plants per m^. These experiments 
point to the conclusion that the amount of irradiance intercepted by 
the canopy is a major determinant of the CGR during the vegetative 
phase of maize growth where nutrients and soil moisture are not limiting. 

Grain yield correlates well with CGR up to an optimum density. 
Decline in yield at high populations is mainly due to barren plants 
(Prine, 1971; Yoshida, 1972; Duncan, 1975). However, an increase in 
grain yield with increase in planting rate normally ceases before 
significant number of barren stalks are found. This yield plateau 
occurs when light interception by the canopy is essentially complete 
so that little increase in photosynthesis per unit land area is 
possible (Prine, 1971; Duncan, 1975). 

The grain yield of a crop is the product of the average rate of 
grain dry matter accumulation per unit area and the effective duration 
of grain filling. The effective period of grain filling (EFP) is 
defined as the quotient of final reproductive dry weight and the average 
rate of grain dry matter accumulation per unit area (Daynard et al . , 
1971). The average reproductive dry-weight accumulation rate can be 
estimated from the slope of a plot of reproductive dry weight against 
time during the linear phase (Johnson and Tanner, 1972b; Duncan, 1973). 

An alternative method of measuring relative differences in the 
duration of the grain filling period involves the use of black-layer 
development and silking date. Black-layer at the base of the nucellus 
of maize kernels coincides with maximum kernel dry weight (physiological 
maturity) and marks the end of the filling period (Daynard and Duncan, 



8 

1969; Daynard et al . , 1971; Daynard, 1972). Thus, total filling period 
can be estimated on a phenological basis as days from pollination to 
black-layer formation. 

A linear relationship has been found among corn cultivars between 
grain yield and duration of the phenological ly estimated total filling 
period (Funnah, 1971), and between grain yield and duration of the EFP 
(Daynard et al . , 1971; Daynard, 1972). In Daynard's experiments, yield 
was more highly correlated with EFP than with the rate of filling. In 
a comparison between inbreds and hybrids, Johnson and Tanner (1972a, 
1972b) found the EFP to be much longer in the hybrids, but their rates 
of growth per unit area were also higher. Several authors (Daynard 
et al., 1971; Peaslee et al . , 1971; Egli and Leggett, 1976) have 
suggested that an extension of the filling period will result in higher 
yields, provided that the rate of dry matter accumulation does not 
change, the grain has larger potential size, and environmental and 
nutritional conditions do not limit yield. 

Estimation of maize grain yield as the product of mean ear growth 
rate multiplied by EFP is convenient, since it evades the assessment 
of the lag phases either at the beginning or at the end of ear growth 
(Duncan, 1975). The initial lag period, from pollination to the 
beginning of EFP, may be important in relation to the number of kernels 
in each inflorescence. Among wheat cultivars, the longer the initial 
lag phase the more kernels there were in each ear (Rawson and Evans, 
1971), especially at low temperatures (Sofield et al . , 1974). However, 
the length of the lag phase in wheat is only a few days, whereas in 
maize it is between 15 to 18 days (Evans, 1975), comprising from 47 to 
84% as long as the effective duration of grain filling (Johnson and 



9 

Tanner, 1972a, 1972b). This much longer duration of the lag period may 
be associated with the presence of 10 to 100 times as many grains per 
inflorescence in maize compared with wheat, and may be necessary to 
allow the later florets in the maize inflorescence to set grains (Evans 
and Wardlaw, 1976). 

Recently, investigations have been reported that give insight into 
which of the parameters, rate or duration, can be most easily modified 
to increase the final yield. Gay et al . (1980) investigated the 
physiological basis for the difference in yield between old, low-yielding, 
and new, high-yielding soybean cultivars in two maturity groups. They 
concluded that the increase in yield between new and old varieties 
has been the result of increased length of the filling period. However, 
Duncan et al. (1978), in a similar study with peanut ( Arachis hypogaea 
L.), assessed equal importance to duration of the filling period and 
growth rates. 

Daynard and Kannenberg (1976) provided results supporting the 
suggestion that selection for a longer EFP represents a feasible means 
of increasing yields of corn; in their experiments, however, dif- 
ferences in filling period accounted for only a limited portion of 
the total variation in yield among 30 hybrids in one experiment and 
35 in another. They also noted that some of the high-yielding hybrids 
had shorter than average filling periods in both experiments. Con- 
trastingly, some of the hybrids had long filling periods and below 
average yields. They speculated that length of filling period and 
yield are set primarily by the size of the ear sink capacity, i.e., 
kernel number, established soon after flowering, and kernel size, 
which is to a large extent genetically controlled. 



10 
With a given rate of grain dry-matter accumulation, larger kernel 
weight may allow a longer filling period and higher yield, assuming 
that supply of assimilates is adequate to satisfy ear demands. 

Duncan et al . (1978) proposed the concept of partitioning, 
defined as the division of daily assimilate between reproductive and 
vegetative plant components, and concluded that the yield increase 
in newly developed peanut cultivars has been the result of higher 
partitioning to reproductive sink. However, the possibility of yield 
improvement from an increase in the partitioning of the photosynthate 
to seed would be reduced when partitioning approaches the maximum 
(Gay et al . , 1980). In the study reported by Duncan et al . (1978) 
the old peanut cultivars partitioned poorly whereas new peanut culti- 
vars were approaching a maximum. In maize, partitioning and daily 
photosynthate supply determine kernel number set during or shortly 
after silking. A relatively large kernel set would be beneficial 
since the sink demand (kernel number times growth per seed) would 
determine the rate of utilization of assimilate, whether from photo- 
synthesis and/or from labile storage (W. G. Duncan, Professor of 
Agronomy, University of Florida, personal communication). However, 
another possibility for establishment of the sink may be by setting 
the kernel size soon after flowering, as Wilson and Allison (1978a) 
suggested; perhaps this occurs once the number of endosperm cells 
has been determined (Wardlaw, 1970). Wilson and Allison (1978b) 
found that the removal of alternate plants in the field had little 
effect on the average weight per kernel of the remaining plants when 
it was done more than two to three weeks after silking, but increased 
final average kernel weight when removal of plants was done close to 



n 

the time of silking. Very similar results have been reported by Prine 
(1971) who worked with semi -prolific maize hybrids. 

The assimilate used for ear growth may come from current photo- 
synthate produced by the canopy or from labile assimilate stored 
earlier in the vegetative component, particularly in the basal inter- 
nodes (Duncan, 1975). Normally, the high rate of photosynthate 
utilization by the growing plant does not permit a substantial accumula- 
tion of assimilates (Duncan, 1975; McPherson and Boyer, 1977); however, 
many studies (Singh and Nair, 1975; Campbell, 1964; Jurgens et al . , 
1978; Hume and Campbell, 1972) have revealed that previously stored 
assimilate can be mobilized and utilized for grain filling, even when 
all leaves were removed and the entire plant was wrapped in foil 
(Duncan et al . , 1965). The results of these experiments have clearly 
demonstrated that assimilates can be translocated from other plant 
components to the ears; however, it is also likely that storage of 
soluble materials occurs when photosynthate exceeds utilization, and 
depletion when the demand is greater than the amount of assimilates 
produced by photosynthesis. Incomplete utilization of photosynthate 
for grain would also occur in cultivars in which the grain matures 
before leaf senesce (Duncan, 1975). In such cases, an assessment of 
the soluble materials accumulated in the stem sap could provide 
useful information about yield-limiting factors. That is, if sink is 
limiting, soluble materials should accumulate during the period of 
active ear filling since assimilate production exceeds utilization. 
On the other hand, if photosynthesis is limiting, soluble materials 
should decline as utilization exceeds supply. Indeed, the decrease 
in stalk weight before and shortly after anthesis and its subsequent 



12 

decrease during ear filling show the storage and utilization of soluble 
solids (Hume and Campbell, 1972; Vietor et al . , 1977; Major et al . , 
1972; Johnson and Tanner, 1972a). 

Normally, in a maize field, there is considerable variation 
among plants in Brix readings of stalk- sap. The Brix reading is an 
expression of the refractometric index using the correspondent percent 
of dissolved sucrose which would give a similar index. Willaman et 
al. (1924), in an intensive study of the possibilities of using corn- 
stalk juice as a source of syrup, found that the stalk-sap of sweet 
corn at the canning stage had a density of 9 to 10° Brix. After 
standing 10 to 20 days following removal of the ears, the stalk-sap 
density was reported to increase up to 13 to 17° Brix, with sucrose 
in some plants reaching as high as 15%. Clark (1913) and Bodea (1934) 
have shown that the expressed juice of maize stalks contains from 
10 to 12% sugar at the time of ear formation and as high as 17% sugar 
if pollination is prevented. Duncan (1975) reported that from corn 
plants selected at random, stalk-sap Brix readings from lower inter- 
nodes ran from 3 to 11%. He suggested that this variation probably 
reflected differences in sink capacity and/or inadequate photosynthetic 
rates, and that stalk Brix readings could furnish diagnostic indica- 
tions, pointing to causes for yield limitations of varieties and 
locations. Van Reen and Singleton (1952) showed that reliable com- 
parisons between varieties can be made using Brix readings. However, 
they also pointed out that caution should be taken, since some 
varieties could store much of the sugars as hexoses rather than 
sucrose, or the concentration of salts and non-sugars components 
may be high. They recommended that in order to apply the hand 



13 

refractometer to other varieties with some assurance as to the meaning 
of the results, sucrose should be determined chemically with at least 
a few samples over the range of Brix values encountered. 

Assimilate remaining in maize stalks at the end of the growing 
cycle represents energy fixed by photosynthesis that was not converted 
into grain, and hence a loss of potential yield, A complication is 
the fact that lodging is negatively correlated with sugar content in 
the stalks (Mortimer and Ward, 1964; Campbell, 1964). Campbell (1964) 
studied three single-cross corn hybrids and showed that structural 
tissues and vascular systems, the insoluble fraction, from unpollinated 
and pollinated plants did not differ significantly, Unpollinated 
plants accumulated more total dry matter in stalks and leaves than 
pollinated plants. The difference in stalk dry matter was readily 
accounted for as soluble solids. His results showed an inverse 
relationship between ear dry matter and soluble solids in the sap just 
before maturation. The prolific, high-yielding, lodging-susceptible 
hybrid maintained a soluble solids level in the stalk juice between 
8 to 10% Brix. Non-prolific, low-yielding, but lodging-resistant hybrids 
gradually attained concentrations of 12 to 14%. Non-lodging unpol- 
linated plants of all hybrids accumulated soluble solids from 15 to 
17% Brix. Campbell concluded that since pollinated and unpollinated 
plants differed primarily in stalk soluble-solids content, this stalk 
component influenced final stalk strength and, therefore, lodging 
resistance. Thus, selection of varieties for resistance to lodging 
operates against complete utilization of assimilates for grain. 

Photosynthesis provides essentially all the increase in crop 
weight and all of the metabolic energy required for crop development. 



14 
The course of photosynthesis is thus a major determinant of crop yield. 
In maize the leaf blades are the main photosynthetic organs. Allison 
(1964) found leaf blades to comprise more than 80% of the total green 
surface at both anthesis and maturity. Crop growth rate depends on 
both the rate of leaf area expansion and the rate of photosynthesis 
per unit leaf area. During early growth, the rate of leaf area 
expansion is of greatest importance; however, once the leaf canopy 
has closed, canopy photosynthetic rate becomes the most important 
determinant of CGR depending on climatic conditions and canopy archi- 
tecture (Duncan, 1971). 

A vertical leaf arrangement is usually desirable in dense crops. 
Maize cultivars generally have a leaf arrangement intermediate between 
horizontal and vertical (Allison and Thomas, 1974). McCree and 
Kenner (1974) felt that the degree of change in leaf angle which is 
likely to be practicable probably would not have a marked effect on 
crop assimilation. Furthermore, optimum leaf arrangement for various 
conditions is far from clear because of the possible influence of 
factors other than the light relations to which attention is usually 
confined (Evans, 1975). Differences in leaf arrangement are probably 
of little importance in maize stands with an LAI of 3 to 4 (Loomis 
and Williams, 1969; Duncan, 1971). 

From a survey of 22 races of maize, ranging from ancient varieties 
to a modern hybrid, Duncan and Hesketh (1968) found no evidence that 
improvement of maize over the centuries has been associated with 
increase in leaf photosynthetic rate. There were, however, differences 
among races in the way their photosynthetic rates responded to 
temperature, which appeared to be adaptive. High-altitude races, for 



15 
example, had relatively lower rates at high temperature, but did not 
differ from low-altitude races at low temperatures. The ancient 
races had net photosynthetic rates near the average for all races. In 
their experiment, only the modern single-cross hybrid ranked con- 
sistently high in photosynthetic rates at all temperatures. Differences 
among maize cultivars in photosynthetic rates have been reported, but 
rate differences appear to be dependent on environmental conditions. 
Rate differences found by Heichel and Musgrave (1969) in the Philippines 
were not always apparent at Cornell University at Ithaca, New York 
(Gifford, 1970). Similarly, although heterosis in photosynthetic rates 
was reported by Heichel and Musgrave (1969) and Derieux et al . (1973), 
there are other studies where it was not found (Duncan and Hesketh, 
1968). 

Apparent photosynthesis (Shibles, 1976) has been shown to decrease 
as leaves age. Vietor et al. (1977) studying the effect of leaf age 
on photosynthetic rates in maize showed decreases in apparent photo- 
synthesis of individual leaves at different positions in the stalk. 
The study was done in greenhouse and field cultured open pollinated 
varieties, inbred lines, and single-cross hybrids. Mean apparent 
photosynthetic rates across leaf position at pre-tassel , silking, and 
dough stages for one inbred line were 52.8, 39.1, and 20.0 mg CO2 
dm-2 h"^, respectively. These measurements were done under greenhouse 
conditions. In an open-pollinated, field grown cultivar, mean apparent 
photosynthetic rate at the same stages as above and across leaf posi- 
tions were 58.1, 45.0, and 28.7 mg CO2 dm-2 h"^. Pre-tassel, silking, 
and dough stages were at 44, 71, and 88 days after emergence, respec- 
tively. Their measurements of apparent photosynthesis in eight 



16 
single-cross hybrids at the silking stage ranged from 68.7 to 55.2 
mg CO2 dm-2 h-^. The silking stage was recorded 71 days after 
emergence; the measurements were done on the leaf immediately above 
the ear. At the dent stage, 110 days after emergence, the range was 
48.7 to 36.9 mg CO2 dm-^ h-^. Leaf area index at the silking stage 
ranged from 4.31 to 4.12, and at the dent stage from 4.08 to 3.99. 
Thus, their study showed a reduction in mean apparent photosynthesis 
of more than 62% in inbred lines, 50% in open pollinated varieties, 
and 32% in single-cross hybrids from the early stages to the latest 
one. They attributed the decrease in mean apparent canopy photosynthesis 
to the decrease in mean leaf photosynthesis as caused by leaf aging. 

Crosbie et al . (1977) also have shown a decrease in leaf photo- 
synthetic rate from the vegetative to the reproductive stage. They 
studied photosynthetic rate variability in 64 inbred lines, which were 
selected randomly from a set of 247 inbred lines developed from Iowa Stiff 
Stalk Synthetic (BSSS) population. They showed decreases of 30% in 
the mean CO2 exchange rate (Shibles, 1976) from measurements performed 
at stage 3.5 (twelfth leaf completely visible) to measurements taken 
at stage 6.0 [12 days after silking or "blister" stage (Hanway, 1971)]. 
At stage 3.5 they measured 36.6 mg CO2 dm-^ h"^ from the most recently 
expanded leaf. At stage 6.0 they measured 25.7 mg CO^ dm-^ h-^ from 
the second leaf below the tassel. 

Wilhelm and Nelson (1978b) studied leaf growth, leaf aging, and 
photosynthetic rates in tall fescue ( Festuca arundinacea Schreb) 
genotypes. The genotypes were selected (Wilhelm and Nelson, 1978a) 
to represent four carbon-exchange rate-yield categories: high CER-high 
yield, high CER-low yield, low CER-high yield, and low CER-low yield. 



17 
Their experiment showed that CER of all four genotypes decreased after 
collar formation at a rate of about 15 to 20% per week. They grew one 
crop in the greenhouse during a four week period and another in the 
field during a six week period. The CER measurements were done in 
leaves previously marked and the rates were measured at an approximate 
interval of one week on clear days. 

Another possible reason for the decrease in photosynthetic rates 
can be ascribed to increases in nonstructural carbohydrates (NSC) in 
the leaves. Decreased photosynthetic rates with a concurrent increase 
in NSC were observed in soybean following pod removal (Mondal et al., 
1978). On the other hand, an increase in photosynthetic rates of 
leaves during fruit formation has also been observed in soybeans 
(Dornhoff and Shibles, 1979; Ghorashy et al., 1975). This effect has 
been hypothesized to be due to an increased demand for photosynthates 
by the developing fruit with a resulting decrease in NSC. Decreased 
NSC concentration has been thought to stimulate photosynthesis by 
alleviating end product inhibition by soluble sugars (Neales and 
Incoll, 1968), or by decreasing starch concentration of leaves 
(Nafzinger and Koller, 1976; Thorne and Koller, 1974; Upmeyer and 
Koller, 1973). High starch levels may inhibit net leaf photosynthesis 
by starch granules physically shading the chloroplasts, by increasing 
biochemical carboxylation resistance, or by increasing the CO2 
diffusion pathlength due to physical swelling of chloroplasts 
(Nafzinger and Koller, 1976). 

Hageman et al. (1976) reviewed their attempts to screen and select 
corn varieties for activity of specific essential enzymes (aldolase, 
glucose 6-phosphate dehydrogenase, trisephosphate dehydrogenase, and 



18 
nitrate reductase). Genetic differences in activities of single enzymes 
were found among cultivars; however, these did not provide a metabolic 
explanation for hybrid vigor in dry matter production, because crosses 
did not exhibit heterotic enzyme activity when compared with the 
parental inbreds. On the basis of their failure to find single enzyme 
activities to account for heterosis, they suggested that an efficiently 
organized total metabolic system is the characteristic of a superior 
corn variety. 



MATERIALS AND METHODS 

This study was conducted at the Agronomy Farm of the University 
of Florida during the 1978 and 1979 growing seasons. The soil used 
in 1978 was a poorly drained, loamy, silicious, hyperthermic Arenic 
Paleudult (Kendrick loamy sand). In 1979, the soil was a taxajunct 
classified as loamy, mixed, thermic Arenic Hapludalf ( Jones ville 
fine sand) with excellent drainage. 

Rainfall, solar radiation, and maximum and minimum temperatures, 
collected near the plots at the Agronomy Farm Weather Station, were 
averaged over 10-day periods coinciding with harvest dates. These 
data for 1978 and 1979 are presented in Figs. 1 and 2. Anthesis dates 
were recorded in both years when 50% of the plants in each plot were 
shedding pollen. General information on fertilizers and pesticides 
is presented in Table 1. 

In 1978, the experiment was seeded on June 9. The corn cultivars, 
single-cross hybrid Pioneer Brand 3369A and inbred line Iowa B37, were 
planted in a complete randomized design with four replications. The 
seeds were hand planted on square spacing of approximately 45 cm for 
the hybrid and 30 cm for the inbred. Two or three seeds were placed 
at each planting point to insure uniform stands. Fifteen days after 
emergence the plants were thinned to one per planting point. Final 
plant populations were 4.8 and 10.8 plants per m^ for the hybrid and 
inbred, respectively. Nitrogen was applied 30 days after planting at 

19 



20 



MJ/m 
24-1 

19 

14 



35 

32 

29 

26H 

23 

204 



SOLAR RADIATION 




TEMPERATURE 




MAX. 



MIN. 



1H- 



mm 
20- 

15- 
10 

5^ 



Va 



4- 



PRECIPITATION 



13 23 33 43 53 63 73 1 
DAYS AFTER PLANTING 



8 



Figure 1. Climatological data for the 1978 experimental period. 
Averages of 10-day periods coinciding with harvest 
dates. 



21 



MJ/m 
25 

20 

15 



•^Y 



°C 
34 

22 

10 



SOLAR RADIATION 




I I I I 



TEMPERATURE 



■ I 




^^r 



I I I I < I I I 



mm 
40- 



20 



^^T 



PRECIPITATION 



25 45 65 85 
DAYS AFTER PLANTING 



105 



i: 



Figure 2. Climatological data for the 1979 experimental period, 
Averages of 10-day periods coinciding with harvest 
dates. 



22 



Table 1. Fertilizers and pesticides used in the maize growth analysis 
experiments of 1978 and 1979. 



Product 


Rate kg/ha 




Application Date 




1978 Experiment (9 June through 


25 August) 


Fertilizers: 
Dolomite 
4-8-16 
Nitrogen 
Nitrogen 


2000.0 

600.0 

80.0 

30.0 




1 month before planting 
1 month before planting 
30 days after planting 
45 days after planting 


Nematicides: 
Carbofuran 
DBCP 


1.5 ai 
12.0 ai 




1 week before planting 
1 week before planting 


Herbicides: 
Butyl ate 
Atrazine 


3.0 ai 
2.0 ai 




Pre- emergence 
Pre-emergence 


Insecticide: 
Carbaryl 


0.5-1.0 1/ha ai 




When necessary 




1979 Experiment (21 March 


through 23 July) 


Fertilizers: 
4-8-16 
Nitrogen 
Nitrogen 


600.0 

- 40.0 

40.0 




10 days before planting 
30 days after planting 
45 days after planting 


Herbicides: 
Butyl ate 
Atrazine 


3.0 ai 
2.0 ai 




10 days before planting 
10 days before planting 


Nematicides: 
Carbofuran 


1.5 ai 




At planting 


Insecticide: 
Carbaryl 


0.5-1.0 1/ha ai 




When necessary 



23 
different rates because of the different plant populations. The 
densities used gave similar LAI values for both cultivars throughout 
the critical part of the experimental period. 

Sampling began 23 days after planting and continued at intervals 
of 10 days until day 73. The last sampling at day 78 had a 5-day 
interval. Samples were taken from each plot beginning at one end and 
moving successively across the field, leaving one or two border rows 
between sampling sites. The plants were removed from the soil with 
a shovel to insure extraction of most of the root weight. From the 
third harvest until the last sampling date, the roots were left in the 
soil on the assumption that minimum additional root growth was taking 
place (Hanway, 1963, 1971; Whaley et al., 1950). 

Total dry matter production of both hybrid and inbred cultivars 
was calculated for harvests 3 to 6 by adding the root weight of 
harvest 2 to the other plant components on each respective harvest 
date. Six harvests were taken, each consisting of three subsamples 
as follows: 

i. Ten plants per plot were harvested to measure the total 
fresh weight. Ear fresh weight was determined separately. 

ii. An average plant was selected from each plot and used to 
determine fresh and dry weight of plant components: roots, stalk 
(stem and sheaths), leaves, and ears. These plant components were 
dried at 60° C for three days. Leaf area was measured with a photo- 
electric planimeter (Hayashi Denko Co., Ltd., Automatic Area Meter, 
Type AAM-5) and used to calculate LAI. The ratio dry weight to fresh 
weight of the whole plants, and of the ears, was used to estimate 
total as well as ear dry weights of the 10-plant sample. 



24 
iii. Six plants from each plot were sampled to estimate soluble 
solid concentrations in the internodes. An American Optical to 25% 
Brix hand refractometer was used. The Brix readings were measured in 
the second, fourth, sixth, and eighth internodes, beginning from the 
base. Brix readings were taken until day 83. 

In 1979, four cultivars were studied: Coker 77, a closed-pedigree, 
high yielding hybrid; Maiz Criollo, a racial accession from Cuba; and 
Chapalote and Nal-Tel, two ancient Mexican races. Chapalote and Nal-Tel 
are races believed to have arised from primitive sources without 
hybridization (Wellhausen et al . , 1952). The experiment was hand-planted 
on March 21 in a split-plot arrangement with four replications. Ten 
main plots, each representing a harvest date, were arranged in such a 
way that two border rows of Coker 77, planted continuously in the 
middle of the experimental area, separated each replication into two 
sets of five main plots. Additionally, two border rows of Coker 77, 
in the outside of the main plots, were planted to insure equal competi- 
tion among cultivars. The sub-units consisted of two rows per cultivar 
without border separation between sub-units in the same main plot. 
Each sub-unit contained 24 planting sites. One or two seeds were placed 
at each planting site. Twenty plants were used for sampling and two 
plants, each at the row's ends, were utilized as guard plants between 
sub-units of the other main plots. Missing sites were replanted with 
plants of equal age and size, resulting in nearly perfect stands for 
all cultivars. Final plant population was 4.3 plants per m^. Harvest 
dates and cultivars were randomly assigned to main plots and sub-units, 
respectively, by means of a table of random numbers. Overhead irrigation 



25 

was applied at planting to encourage uniform germination and provide 
adequate soil moisture during periods of low rainfall. 

Sampling began 34 days after emergence. The second sampling 
was made 11 days later. Subsequent sampling continued at 10-day 
intervals until 125 days after planting with a total of 10 harvests. 
Each sample consisted of three sub-samples as follows: 

i. Five plants were selected from each sub-plot and used to 
estimate the internode density in Brix readings. The readings were 
made on alternate internodes beginning in the second internode at 
ground level. Juice from the internode was obtained by squeezing the 
basal end with pliers. Several drops of juice were placed onto the 
glass surface of an American Optical and 25% Brix hand refracto- 
meter for its reading. Readings were performed only for the main 
stalk. 

These five plants were also used to determinfthe percent dry 
matter distribution of vegetative and reproductive plant components. 
A plant consisted of the main stem plus any tillers. From the ears, 
a random sample of 100 kernels was weighed to estimate the average 
kernel weight (AKW). From the plot of AKW and time the rate of 
kernel dry matter accumulation (KGR) was estimated using linear 
regression analysis (Steel and Torrie, 1960). Effective seed filling 
period (ESFP) was calculated by the ratio AKW to KGR. 

Stalk (stems and sheaths), leaf blades, cobs, and kernels were 
ground in a Wiley mill with 1-mm screen. Cobs and kernels were ground 
again to pass through a 40 mesh screen. Total available carbohydrates 
(TAG) were determined in the ground material by the procedure outlined 
in Appendix B. The fractions included in the TAG determination are: 



26 
the reducing sugars, glucose and fructose; the transport sugars which 
are nonreducing, mainly sucrose; and the storage carbohydrates, mainly 
sucrose and starch. 

ii. Ten plants in each sub-unit were harvested and dried in a 
convection oven at 60° C for four days. The measured total dry weight 
was added to the total dry weight of the five-plant sample, such that 
total dry weight of 15 plants was used to calculate the CGR and ear 
growth rate (EGR) of each cultivar. Dry matter distribution of the 
15-plant sample, in grams per m^ and for each variety, was estimated 
by multiplying its total dry weight by the percent dry matter calculated 
from the 5-plant sample. 

iii. A representative plant from each sub-unit was sampled for 
leaf-area determination. Leaf area was measured with a Lambda Instru- 
ment Corp., LI-3100 Area Meter, and utilized -to estimate LAI. 

Total available carbohydrates, in grams per m^, were calculated 
by multiplying each component (stalk, leaves, cobs, and grain) of 
the 15-plant sample by the percentage TAC in each part. 

Crop growth rate and EGR for each cultivar were calculated by 
simple linear regression analysis applied during the linear phase of 
the 15-plant sample total and ear dry-matter accumulation curves 
(Steel and Torrie, 1960). 

/ Effective ear filling period (EEFP) was estimated by the quotient 
of final yield and EGR. Number of kernels per m^ was calculated by 
dividing the product of ear yield times shelling percentage by AKW. 

In all rates of dry matter accumulation determinations, separate 
linear regression equations were computed for each replication. The 
slopes of these lines were used as treatment variables for statistical 



27 
analysis of dry matter accumulation rates. AlJ_characteri sties 
observed or calculated were tested by analysis of variance and ranked 
by Duncan's Multiple Range procedure at the 0.05 level of probability. 
Statistical analyses were done with the use of SAS 79.3 Statistical 
Analysis System by Barr et al. (1979). Analysis of variance and 
regression analysis were performed using the ANOVA (analysis of 
variance) and the GLM (general linear models) procedures^ 



RESULTS AND DISCUSSION 
1978 Experiment 
Vegetative Yields 

Late planting in 1978 together with poor soil drainage resulted 
in unfavorable conditions for a proper development of both the hybrid 
and inbred cultivars. Prine and Schroder (1965) showed that late 
planting decreased the growth cycle of the semi-prolific hybrid Florida 
200 by more than 30 days from planting to 50% silking. They found 
that yield for maize planted on May 8 was 40% lower than that planted 
March 8. In 1978, rainfall during the month of June was adequate for 
crop growth. However, rainfall in July and August was 30% and 17% 
higher, respectively, than the 70-year rainfall average for the same 
months in the Gainesville area (McCloud, 1979). The high rainfall 
in July and August, poor soil drainage, and high temperatures caused 
stunted plants and shortened the growth cycle of the cultivars (Valle, 
1978). A high relative humidity also increased leaf, stalk, and ear 
diseases, which severely decreased yields (Valle, 1978; Chapman et 
al., 1978). 

Vegetative yields were taken throughout the growing season (Fig. 
3). Differences in total dry weight between cultivars at day 23 were 
not statistically significant. However, from day 43 through 78 total 
dry weights for the hybrid were significantly greater than for the 
inbred. The stalk dry weight of the hybrid increased to a maximum of 

28 



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30 
337.3 gm-^ at day 73 and then declined. Peak stalk dry weight for 
the inbred was 279.0 gm"^ at day 63. Leaf dry weight increased for 
both cultivars until day 43. Both cultivars maintained a plateau in 
the leaf weight component from day 43 to 73. Maximum leaf weight at 
day 73 for the hybrid and inbred were 157.8 and 122.0 g m"2, respectively. 
Statistical significances (p = 0.05) in the leaf dry weights between 
the two cultivars were found from day 43 until the end of the experi- 
ment. 

Pollination occurred at day 53 for the hybrid and 55 for the 
inbred. Maximum dry ear weight for the hybrid was 30.5 g m"2 at day 
78. From day 63 to 73 (the period of linear ear growth) the hybrid 
accumulated 34% in the stalks of the total dry weight produced in 
this period, 52% in the leaves, and 14% in ear growth. There was no 
reproductive component in the inbred. 
Crop Growth and Ear Growth Rates 

The rate of dry matter accumulation (CGR) was calculated for both 
cultivars from day 23 to 63 after planting. The CGR values for hybrid 
and inbred were 12.2 and 9.1 g m"^ day"^, respectively. These CGR values 
were significantly different at the 0.05 level of probability when 
ranked by Duncan's Multiple Test procedure. Therefore, with the data 
obtained and the conditions of this experiment, mean CGR values for 
hybrid and inbred compared at nearly equal LAI were significantly 
different, indicating differences in net photosynthesis between them. 
However, in an earlier study with the same cultivars, Valle (1978) was 
unable to detect a statistically significant difference for the CGR's 
for the before anthesis period, although significant differences were 
found after anthesis. 



31 
The EGR, the rate at which the ear was filled, was computed for 
the hybrid from day 63 to 73 and is expressed by the equation: 
Y = -16.6 + 0.64 X. 
Brix Readings 

Since by day 55 it was apparent that the hybrid and the inbred 
line would produce abnormally low yields, it was decided to investigate 
the level of soluble solids accumulated in the stalks. The hypothesis 
was that photosynthate that could have gone to the ear would accumulate 
mainly in the stalk. Data presented in Table 2 show that the mean 
Brix readings per plot for the hybrid and the inbred at day 83 after 
planting (columns 9 and 10) were higher for the inbred line. The means, 
compared using a t-test, were significantly different at the 0.05 
level of probability. These results tend to indicate that the photo- 
synthate produced by the inbred line, which could have filled the ear, 
was accumulated in the stalk to a much greater extent than in the 
hybrid corn; this suggested that the sink capacity of the inbred was 
lower. 

1979 Experiment 
Cultivar Characteristics 

The cultivars studied during the 1979 growing season were chosen 
because of their similar characteristics when grown in Florida (Table 
3). The similar height of the cultivars facilitated the field layout 
used, since seeds of the ancient races, Chapalote and Nal-Tel, as 
well as Maiz Criollo were scarce. 

Emergence of all cultivars began approximately one week after 
planting. Stands in all plots were nearly perfect; however, skips 



32 



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0) 


ro 


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33 



Table 3. Characteristics of the cultivars studied in the 1979 growing 
season. t 



Cultivar 


Plant 
height 


Ear 
height 


Leaves 


Ears 


Tillers 


Anthesis1= 








number/plant 




Chapalote 


2.8 


1.4 


12.7 


1.8 


2.0 


66 


Coker 77 


2.9 


1.4 


15.4 


1.6 


0.1 


72 


Maiz Criollo 


2.8 


1.6. 


16.0 


1.1 


0.1 


71 


Nal-Tel 


2.6 


1.4 


14.2 


1.8 


0.3 


65 



t Mean of 30 plants. The measurements were done 125 days after 
planting. 

t Days from planting. 



34 
were replanted with seedlings of similar age and size. Anthesis for 
Chapalote, Coker 77, Maiz Criollo, and Nal-Tel occurred at day 65, 71, 
72, and 66, respectively (Table 3). These dates closely agreed with 
unpublished results for 1978 of Dr. E. S. Horner (Professor of Agronomy, 
University of Florida). The early maturing Chapalote had an average of 
two tillers per plant at the end of the growing season as compared with 
few or none for the other cultivars. The late maturing Coker 77 and 
Maiz Criollo produced approximately two or three more leaves than the 
other two cultivars. The cultivars began to show signs of senescence 
(dead lower leaves) in the period between days 95 and 105 in Chapalote 
and Nal-Tel, and at day 115 and 125 in Maiz Criollo and Coker 77, 
respectively. A high LAI in Coker 77 and Maiz Criollo was maintained 
for longer periods than in Chapalote and Nal-Tel. The LAI of Chapalote 
was similar to that of Coker 77 during its reproductive phase (Fig. 4). 
Nal-Tel had the lowest LAI. 
Total Dry Weight 

Total dry matter accumulation in all cultivars followed a typical 
sigmoidal curve (Fig. 5). Total dry weights were not statistically 
different among cultivars for the first 55 days of crop growth. By 
day 65, Chapalote, Maiz Criollo, and Coker 77 had produced more dry 
weight than Nal-Tel. However, the dry weight of Coker 77 and Maiz 
Criollo did not differ statistically from that of Nal-Tel. Total dry 
weights for Chapalote and Coker 77 at day 75 were significantly greater 
than those of Maiz Criollo and Nal-Tel due to higher weights of their 
stalk and leaf components. 

From day 85 until the end of the experiment, the total dry weight 
of Coker 77 significantly differed from the other cultivars. This 



35 














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36 




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37 

difference was mainly the result of its high ear dry weight. During 
this period Nal-Tel had consistently lower total dry weights. Chapalote 
and Maiz Criollo were intermediate. 

Peak dry weight for Coker 77 was 2476 g m-^, recorded at day 115. 
Chapalote, Maiz Criollo.and Nal-Tel had peak dry weights of 1838, 1848, 
and 1563 g m-^, respectively, which were measured at day 95 in Chapalote, 
day 105 in Nal-Tel, and day 125 in Maiz Criollo. While Chapalote, 
Coker 77, and Nal-Tel showed decreased total weight at day 125, Maiz 
Criollo showed increases as a result of ear weight increase. 
Root Dry Weight 

Root dry weight increased in all cultivars until day 75, after 
which a plateau was maintained throughout the experimental period. 
Average root dry weights during the plateau were 123, 193, 163, and 81 
g m-2 for Chapalote, Coker 77, Maiz Criollo, and Nal-Tel, respectively. 
Mean root weight of Coker 77 was significantly greater than those of 
Chapalote and Nal-TeT. 
Stalk Dry Weight 

Dry weight in the stalk (Fig. 6) differed significantly among the 
four cultivars. Peak stalk weights for Chapalote, Coker 77, Maiz 
Criollo, and Nal-Tel were 980, 1003, 873, and 855 g m"2, respectively. 
After day 85 all cultivars showed a decline in stalk weight, but the 
degree of decrease varied widely among them. The rate of decline in 
Chapalote was markedly more pronounced than in the other cultivars. 
Stalk dry weight in Nal-Tel declined rapidly after day 85 but less 
rapidly than in Chapalote. 

Decreases in stalk weight occurred at the initiation of the ear 
linear phase which began at day 85 in Coker 77, Maize Criollo, and 



38 



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39 
NaT-Tel. However, Chapalote increased stalk weight until day 85, but 
its linear ear growth phase began at day 75. Since Chapalote tillered 
profusely (up to 12 tillers were counted in several plants before 
and after anthesis) the increase in stalk dry weight was likely the 
result of tiller growth. Nal-Tel continued increasing stalk weight 
from anthesis to the beginning of its linear ear growth. This period 
was longer than those of the other cultivars. The continued stalk 
growth into linear ear fill reduced the amount of photosynthate 
available for ear growth. The decrease in stalk dry weight after day 
85 in all cultivars is assumed to be due to translocation of assimilates 
to the growing kernels. 
Leaf Dry Weight 

Leaf dry weight in all cultivars increased rapidly until new leaf 
formation was completed at about day 65 (Fig. 7). After day 65 Coker 77 
had the highest leaf weight, whereas Nal-Tel had the lowest; for the 
latter even the slope of its linear increase leaf weight phase was 
distinctly different. • Peak leaf-weight components were 345, 350, 300, 
and 217 g m-^ for Chapalote, Coker 77, Maiz Criollo, and Nal-Tel, 
respectively. From day 65 the leaf component of Chapalote, Coker 77, 
and Maiz Criollo formed plateaus for periods varying from 20 to 50 days 
until leaf senescence started; Coker 77 had the longest and Chapalote 
the shortest plateau. After reaching its peak weight at day 75, the 
leaf dry weight of Nal-Tel showed a steady decrease until the end of 
the experiment. The decline in leaf weight was more pronounced in 
Chapalote and Maiz Criollo than in Coker 77 or Nal-Tel. 



40 




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41 

Ear Dry Weight 

Dry matter in the ear component differed significantly among 
cultivars (Fig. 8). From day 85 throughout day 125, Coker 77 had a 
significantly greater ear weight than the other cultivars. The ear 
weight of Chapalote was the lowest from day 105 until the experiment 
ended, while Maiz Criollo and Nal-Tel were between the two extremes. 
Maximum ear weights were 1146, 578, 776, and 607 g m"^ for Coker 77, 
Chapalote, Maiz Criollo, and Nal-Tel, respectively. Ear dry weight 
of Coker 77 at the final harvest was 1023 g m"^. 
Dry Matter Distribution 

Root, stalk, leaf, and ear computed as a percentage of the cul- 
tivars' total dry weight are presented in Table 4. Root percentages 
were higher in the vegetative phase. The decrease in root percentage 
during the reproductive phase was the result of root dry weight 
plateaus from anthesis, or shortly after, until the end of the 
experiment, while the total dry weight of the cultivars continued to 
increase. From day 65 to day 125 the percentage of total dry matter 
comprising the stalk component was lower in Coker 77 and Maiz Criollo 
than in Nal-Tel or Chapalote. The stalk component percentage of 
Coker 77 was the lowest. Stalk component percentages of Chapalote 
and Nal-Tel were higher than that of Coker 77 and Maiz Criollo due 
to continued growth of tillers in Chapalote and stalk weight increase 
in Nal-Tel between day 65 and 85. 

Coker 77 had higher ear percentages than the other cultivars; 
Coker 77 had the highest and Chapalote the lowest. From day 85 to 115 
Maiz Criollo had the highest leaf component percentage ranging from 
13.3 to 19.5%; it was also the variety with more leaves (Table 3). 



42 




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44 

Although Chapalote and Nal-Tel tasseled at day 66 and 65, respec- 
tively, the high vegetative component weight up to or well into the 
ear linear phase is evidence that these cultivars continued distributing 
photosynthate to vegetative growth. Stalk and leaf component 
percentages were higher in both cultivars at day 75 and 85 than for 
Coker 11 and Maiz Criollo. Coker 77 and Maiz Criollo, in spite of 
their later anthesis at day 72 and 71, respectively, invested a lower 
fraction of their assimilates into their stalk and leaf components 
before and after anthesis, and began linear increase in ear weight 
sooner than the ancient races. In other words, Coker 77 and Maiz 
Criollo set kernel number and began to fill them with less apportioning 
of photosynthate to vegetative growth. 
Crop Growth Rate 

The early exponential phase of the total dry matter sigmoidal 
curve covered the first 45 days of crop growth (Fig. 5). The cultivars 
increased dry weight at a linear rate from approximately day 45 to 
day 85. Vegetative crop growth rates (CGRv) were calculated from day 
45 to day 65 in Chapalote and Nal-Tel, and from 45 to 75 for Coker 77 
and Maiz Criollo. These periods correspond to beginning of linear 
total weight increase to anthesis. 

/^ Crop growth rates during the vegetative phase for Chapalote, 
Coker 77, Maiz Criollo, and Nal-Tel are presented in Table 5. These 
values are the slopes of the respective linear regression equations. 
I Coefficients of determination for each equation were greater than 
\i 0.986. The difference in CGRv between Nal-Tel and the other three 

cultivars is attributed to the lower LAI of Nal-Tel (Fig. 4). It has ^^ 
been shown that the rate of dry matter production in corn increases "? 



45 

with increasing LAI and percent light interception (Hanway, 1962; 
Ragland et al., 1965; Williams et al., 1965a, 1968). Chapalote, 
Coker 77, and Maiz Criollo had comparable LAI values during most of 
the experimental period. 

Since the CGRv values for Chapalote, Coker 77, and Maiz Criollo 
were not statistically different, presumably, they also had very 
similar canopy photosynthesis and potential to produce high yields. 
Crop growth rate measured from the beginning of the linear phase prior 
to ear development reflects the rate of net photosynthesis, and hence, 
assimilate production rate that could be available for ear growth. 
This assumes, however, that the photosynthate produced would be used 
exclusively to fill grain, that the rate does not change during grain 
filling, and the sink capacity is adequate to utilize it. 

From the beginning of linear increase in ear weights the rates 
of total dry matter accumulation decreased for all cultivars. Crop 
growth rates during linear ear growth (CGRr) for Chapalote, Coker 77, 
Maiz Criollo, and Nal-Tel are presented in Table 5. Coefficients of 
determination for the linear regression equations explained more than 
90% of the variability in the sum of squares for total dry matter. 
The CGRr values for Chapalote and Coker 77 were not statistically 
different from each other, but were significantly different from the 
CGRr of Maiz Criollo and Nal-Tel. 

The decrease in total biomass growth rate during the reproductive 
period could have been caused by a decrease in photosynthesis with 
decrease in solar radiation, light interception, leaf aging, or because 
more photosynthate was required in the production of dry matter in the 
seed than in the vegetative portion. This last reason, although 



46 



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47 
important, may not have been the cause of the decrease 1n total biomass 
growth rate, since the vegetative portion of a corn plant has a chemical 
composition similar to the seed (Morrison, 1947; Sinclair and de Wit, 
1975). The energy required to produce a unit of seed weight or a unit 
of vegetative component per gram of photosynthate is nearly equal. 
These relationships are presented in Table 6. 

Production value (Penning de Vries et al , , 1974) is defined as 
the weight of the end product divided by the weight of the substrate 
required for its formation. The production value can be used to 
determine the amount of glucose required for synthesis of plant com- 
ponents provided that the chemical composition is known. For example, 
one gram of maize vegetative matter requires 1.09 grams of glucose while 
one gram of seed and one gram of cob require 1.29 and 1.05 grams of 
glucose, respectively. Using a shelling percentage of 83, the ear yield 
component of the plant requires 1.25 grams of glucose. The ratio of the 
glucose required for the yield component to the vegetative component 
is 1.15. The 15% increase in photosynthate required to produce the ear 
component accounts for only a small part of the decrease in total bio- 
mass production in the reproductive phase. 

A decrease in solar radiation was an improbable cause for the 
decrease in total biomass growth rate during reproductive growth, since 
the average daily irradiance during reproductive growth, June and 
July, was the highest of the experimental period (McCloud, 1980). 
Leaf area index maintained plateaus during most of the vegetative and 
reproductive growth (Fig. 4). Thus, light interception can be assumed 
the same in both periods. 



48 



Table 6. Estimation of glucose required for synthesis for the 
vegetative, seed, and cob components of a maize plant. 



Component 


% 


PVt 


Glucose required 
for synthesis 






g g-^ 




g g"^ 


Vegetative 1= 










Carbohydrate 


80 


0.853 




0.938 


Protein 


6 


0.620 




0.097 


Lipids 


2 


0.351 




0.057 


Grain § 










Carbohydrate 


84 


0.853 




0.985 


Protein 


10 


0.620 




0.161 


Lipids 


5 


0.351 




0.142 


Cobst 








- 


Carbohydrate 


86 


0.853 




1.008 


Protein 


2 


0.620 




0.032 


Lipids 


0.4 


0.351 




0.011 



t Production Value (Penning de Vries et al., 1974) 

t Chemical composition from Morrison (1947) 

§ Chemical composition from Sinclair and de Wit (1975), 



49 
Most of the decrease in crop growth during ear development (CGRr) 
may have resulted from a decrease in apparent photosynthesis or CER 
(Shibles, 1976) as Vietor et al. (1977), Crosbie et al . (1977), and 
Wilhelm and Nelson (1978b) have shown. These studies showed decreases 
in photosynthetic rates with leaf aging sufficient to account for the 
decrease in growth rate (CGRr) during the reproductive phase measured 
in this experiment. There was also rain deficit during the grain 
filling period. Irrigation as supplied may not have been as uniform 
as expected; dry soil caused wilting and hence reduction in photo- 
synthesis. 

It should be recalled that the increase in dry weight in Chapalote 
during the vegetative, and part of the reproductive growth phases, 
was the result of main stalk growth as well as growth of tillers 
most of which were barren. Thus, CGRv as a measurement of dry matter 
increase for crop growth that could be available for ear growth would 
be applicable only to varieties which allocate photosynthate more 
completely to ears. The high CGRv of Chapalote was a composite of 
potentially available photosynthate for ear growth and photosynthate 
invested into tiller growth. The CGRr of Chapalote thus decreased 
either because photosynthesis decreased as leaves aged or it may have 
decreased because of lack of sink. 

The change in growth rate for the period after anthesis and during 
linear ear growth may be exemplified better in Maiz Criollo. This is 
a cultivar that did not tiller and had a high CGRv not statistically 
different from that of Chapalote or Coker 77. However, its total dry 
matter accumulation rate changed drastically during the period of 
reproductive growth. This reduction in growth rate, as stated above. 



50 
likely could have resulted from a decreased canopy photosynthesis which 
assimilate production had to be diverted into two sinks, i.e., ear 
growth and stalk growth, as did Chapalote. However, the decrease in 
dry weight for the leaf and stalk components after day 95 and 105, 
respectively, suggests remobilization and translocation of assimilates 
for ear growth (Figs. 6 and 7). 

These considerations are also applicable to Coker 77 and Nal-Tel, 
although these cultivars did not continue increasing stalk weight after 
the beginning of linear ear growth. Coker 77 had almost double CGRr 
which was maintained for a longer period and also less translocation 
of assimilates to the ear than the other cultivars, as is evident by 
its lower decrease in stalk and leaf dry weights during the reproductive 
period. 
Ear Growth Rate 

The cultivars began to increase ear dry weight at a linear rate 
around day 75 for Chapalote and day 85 for the other cultivars (Fig. 8). 
Ear growth rates were computed using linear regression analysis (Steel 
and Torrie, 1960). These rates are presented in Table 5. The EGR 
of Coker 77 was significantly greater than the EGR of Chapalote, 
Nal-Tel, and Maiz Criollo, while the latter three did not differ 
significantly from each other. 

The EGR is the rate at which the ear is filled. The CGRv, 
calculated prior to ear development, is taken as an estimation of the 
rate of potentially available photosynthate for ear development. The 
quotient of EGR by CGRv is an estimate of the crop's relative increase 
in ear weight as compared to weight increases in the vegetative 
fraction, i.e., an estimation of distribution of dry matter for ear 



51 

growth. These distribution indices (DIv) for Chapalote, Coker 77, Maiz 
Criollo, and Nal-Tel were 0.51, 0.77, 0.57, and 0.88, respectively. 
This means, for example, that Chapalote produced 51% as much as it 
could have produced had all photosynthate been available at the same 
rate during seed filling as it was during vegetative growth and 100% 
of it were allocated to seeds, assuming that grain growth requires 
equal photosynthetic energy of vegetative growth. 

The distribution index in the reproductive phase (DIr), calculated 
by the ratio of EGR to CGR, is presented in Table 5. Ratios greater 
than 1.0 resulted because of the decrease in growth rate during the 
reproductive phase, probably caused by decline in canopy photosynthesis 
as leaves aged. The DIr ratios may also be interpreted as an indica- 
tion of translocation to the ear of assimilates, previously stored in 
vegetative parts, when the demands of the ear became greater than the 
photosynthate produced. This translocation maintained the ear growth 
rate constant for most of the filling period. 
Ear Effective Filling Period 

The presumed linear period over which the ear increases in dry 
weight until it reaches mature weight is referred to as the effective 
ear filling period (EEFP). It was calculated by dividing the final 
yield by the EGR (Table 7), and hence, is a relative measurement of 
the length of the ear filling period. 

Several studies have shown that lengthening the life of a crop 
increased productivity (Alberda, 1962; Daynard et al . , 1971; Van 
Dobben, 1962). However, the filling period is the period in which 
assimilate is distributed into the yield component of the plant. Thus, 
to increase yield, this is the period that should be lengthened. 



52 



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53 

Duncan et al . (1978) found that differences in the EFP, among several 
peanut varieties and one soybean cultivar, accounted for from 7 to 37% 
of the yield difference among cultivars. Gay et al . (1980) concluded 
that a significant portion of the yield difference between an old, 
low-yielding soybean cultivar and a new high-yielding cultivar was due 
to an increase in the filling period. 

The EEFPsfor Chapalote, Coker 77, Maiz Criollo, and Nal-Tel are 
presented in Table 7. The difference in EEFP between Coker 77 and 
Chapalote explained 17% of their yield difference. The EEFP also 
explained 30% of the difference in yield between Coker 77 and Nal-Tel. 
However, the greater EGR of Coker 77 explained most of the yield 
differences. 
Kernel Growth Components 

The weight of 100 kernels, taken at each harvest during the 
reproductive period, allowed the development of a curve of dry matter 
accumulation for individual seeds (Fig. 9). The mean rate of dry 
matter increase in single kernels (KGR) was calculated during linear 
kernel growth using regression analysis. Single kernel growth rate 
is genetically controlled, with short-term environmental conditions 
having minimal effects on the growth rate (Osafo and Mil bourn, 1975; 
Poneleit and Egli, 1979; Tollenar, 1976; Duncan et al., 1965), and 
as a result of remobilization of assimilates it is relatively independent 
of current photosynthate supply (Duncan et al . , 1965; Poneleit and Egli, 
1979). 

Significant differences were found among cultivars in KGR (Table 
7). Coker 77 had the highest KGR, and Chapalote the lowest. 



54 



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55 

Final average kernel weights (AKW) were significantly different 
among cultivars at the 0.05 level of probability (Table 7). Effective 
seed filling period (ESFP) was also determined for the cultivars (Table 
7). The ESFP is defined as the quotient of average kernel weight and 
the average rate of dry matter accumulation during the linear phase 
of grain growth (Hatfield and Ragland, 1967; Daynard et al., 1971). 
The ESFP is the time it would take for kernel development if development 
had proceeded at the same rate from pollination to maturity. 

The rate of dry weight accumulation in the ear is a direct function 
of the number of kernels actively growing. The increase in ear weight 
occurs at an increasing rate as the number of actively growing kernels 
continues to increase. In corn, the period when actively growing 
kernels are added is short, i.e., no more than 10 days (Duncan et al . , 
1965). The rapid beginning of ear growth in Chapalote indicated that 
the number of actively growing kernels increased rapidly, since a sum 
of linear growth rates by the parts, the kernels, must add to increase 
in linear growth rate by the whole, the ear. The difference between 
EEFP and ESFP in Chapalote seems to be in that the kernels grew rapidly 
at early stages, but after the 95-105 day period the rate of kernel 
weight increase slowed drastically in such a way that after day 95 
total ear dry weight increases were not apparent (Fig. 8). Also, 
Chapalote had a relatively high kernel number per m^, which did not 
fully compensate for its low yield, probably as a result of lower 
kernel weight (Table 7). Slow growing kernels which may result in 
large kernel number (Dreyer, 1980) would not give higher yields per 
se, since the kernel must also have the capacity to grow large. 



56 

Coker 77 had a higher KGR, kernel weight, and kernel number than 
the other cultivars. This would explain its higher yield. 
Total Available Carbohydrates 

The percentages of TAC for the different plant components are 
presented in Table 8. Percent TAC in the stalk decreased linearly in 
all cultivars during the first 55 days of crop growth. Between day 55 
and day 85 percent TAC increased to a plateau which ended at day 105 
in Nal-Tel, day 115 in Chapalote and Maiz Criollo, and day 125 in Coker 
77. Leaf percent TAC also decreased linearly during the first 55 days 
of growth. During the rest of the experiment leaf percent TAC 
increased to a maximum and then decreased at the end of the experiment. 
Percent TAC in the cob component decreased rapidly in Nal-Tel, and 
maintained plateaus, from day 85 to 105 in Chapalote and from day 85 
to 115 in Coker 77 and Maiz Criollo. The percent TAC in the grain was 
higher in Maiz Criollo and Coker 77 than in the ancient races. Coker 
77, the best yielding variety, maintained greater percentage TAC in 
vegetative components (leaf, stalk, and cob) during reproductive growth 
than the other varieties; this may indicate greater photosynthate 
availability. ^ 

Total available carbohydrates (TAC) designates the sum of starch, 
sucrose, and reducing sugars. The TAC are carbohydrates readily available 
to the plant as a source of energy. Stalk and leaf TAC weights were 
calculated by multiplying the fraction TAC (Table 8) by the stalk and 
leaf weights. The cob TAC weight was estimated by multiplying ear 
yield by shelling percentage; the product was subtracted from ear yield 
and multiplied by the cob TAC fraction; TAC weight for the grain was 



57 



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58 

obtained by multiplying ear yield by shelling percentage which product 
was multiplied by the TAC fraction in the grain. 

The TAC weights for stalks of Chapalote, Coker 77, and Nal-Tel 
at day 34 did not differ statistically; however, the stalk TAC of 
Chapalote was not significantly different from that of Maiz Criollo, 
which was the lowest. 

From day 34 to the end of the experiment, Coker 77 had signifi- 
cantly higher TAC weights in its stalk and leaves than the other 
cultivars (Figs, 10 and 11). Peak stalk TAC weight in Coker 77 was 
250 g m-2 at day 85. Coker 77 had one of the slowest remobilization 
rates of stalk TAC. Leaf TAC increased from 3 g m-^, at day 34, to a 
maximum of 30 g m-^ at day 105, decreasing to 18 g m-^ at the final 
harvest. 

Peaks of stalk and leaf TAC for Maiz Criollo were 164 and 15 g m-^, 
respectively, at day 105, decreasing to 85 and 5 g m"^ at day 125. 

Stalk TAC of Chapalote and Nal-Tel peaked at 207 and 163 g m-^, 
at day 105 and 85, respectively. Leaf TAC, in both races, peaked at 
day 75 with weights of 15 and 9 g m'^, respectively. 

Total available carbohydrates in the cob component of Coker 77 and 
Maiz Criollo were significantly higher than in Chapalote and Nal-Tel 
from day 85 throughout the end of the experiment (Fig. 12). The cob TAC 
of Coker 77 was almost double that of Maiz Criollo. Peak TAC weights 
for the cob of Chapalote, Coker 77, Maiz Criollo, and Nal-Tel were 15, 
56, 26, and 10 g m-^, respectively. Peak weights were recorded at day 
85 for Nal-Tel, day 95 for Chapalote and Maiz Criollo, and day 105 for 
Coker 77. From peak dates, TAC in the cob decreased until the end of 
the experiment. 



59 




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62 

Total available carbohydrates in the grain increased rapidly in all 
cultivars. Grain TAC in Coker 77 was significantly higher than grain 
TAC of the other cultivars; Coker 77 was followed by Maiz Criollo and 
finally Nal-Tal and Chapalote (Fig. 13). 

From the beginning of grain TAC increase, at day 85, until maximum 
grain TAC weight was reached, at day 115 in Coker 77 and day 125 in 
Chapalote, Maiz Criollo, and Nal-Tel, they gained 649, 243, 409, and 
277 grams TAC per m^, respectively. Assuming that the decrease in TAC 
in the other components (from their maximum to their minimum weight) 
is translocated to the grain, it would explain 9, 26, 13, and 21% of 
the final yield of the cultivars in the same order. Coker 77 and 
Maiz Criollo had more TAC in leaf and cob than Chapalote and Nal-Tel. 
Apparently high cob TAC may be indicative of a good driving mechanism. 

The assumption that the summation of the decrease in TAC weight 
in the different components is translocated to the ear has to be viewed 
with caution, since it is possible that a fair amount of that TAC could 
have been respired. However, it is worth noting that the decrease of 
TAC in the leaf component of Coker 77 was much lower than in the other 
cultivars. Also, its stalk TAC declined quite slowly. This could have 
resulted from greater photosynthate production. 

The decrease in TAC in the various plant components, assumed to 
indicate carbohydrate translocation to the grain during the reproductive 
phase, was greater as canopy senescence progressed. As mentioned before, 
canopy senescence began at day 95 and 105 in the ancient races, day 115 
in Maiz Criollo, and was appreciable only at day 125 in Coker 77. 



63 








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64 

Brix Readings 

Percent Brix readings in corn cultivars, measured either in sap 
from the whole stalk or internode by internode, could be used as an 
indication of inadequate sink capacity or inadequate photosynthesis. 
It is reasonable to assume that photosynthate that is not translocated 
to the ear should accumulate, mainly in the stalk and then in leaves 
and roots. Also, if photosynthate is decreasing due to stress, the 
ear could withdraw assimilates from reserves to fill its kernels. The 
main storage of assimilates are the lower internodes, because they have 
greater capacity than the top internodes thus containing more of the 
sap in which soluble solids are stored, and because photosynthate 
produced by leaves in the top internodes is rapidly utilized by the 
growing ears. Thus, it would be in the lower internodes that a change 
in Brix should reflect accumulation or withdrawal of assimilates more 
accurately. 

Refractometric readings per plot in the four cultivars studied are 
shown in Fig. 14. The influence of high Brix in the top internodes of 
Coker 77 could be the cause for the higher Brix readings per plot 
during the last part of its growth cycle. However, its lower inter- 
nodes (2nd, 4th, and 6th), as well as the lower internodes of the other 
three cultivars, increased by more than 100% from the vegetative to the 
reproductive phase. 

Average percent Brix readings (APBR) in the different internodes 
are presented in Figs. 15, 16, 17, and 18. The figures show only the 
APBR until the 10th internode. Chapalote's APBR increased rapidly 
until day 85, from which the 6th, 8th, and upper internodes maintained 
plateaus until day 105. The 2nd and 4th internodes continued increasing 



65 




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70 

APBR from day 85 throughout day 125 but at a slower rate. In Chapalote 
as well as in NaT-Tel, the 6th and 8th internodes were usually the 
internodes bearing ears. Nal-Tel APBR in internodes 2, 4, and 6 
increased until day 95, but decreased markedly at day 105. Average 
percent Brix readings for the 6th, 8th, and 10th internodes were very 
similar at day 105 and 115. 

Internode APBR patterns for Coker 77 and Maiz Criollo were similar. 
APBR increased in a linear manner in Coker 77 until day 85. From day 
85 to 95 the APBR of the 2nd, 4th, and 6th internodes decreased 
sharply and level ed-off in the 8th. From day 95 until the end of the 
experiment all the internodes of Coker 77 increased APBR. The ears of 
Coker 77 and usually one ear in Maiz Criollo were located generally 
in the 8th and/or 10th internode. The difference between the pattern 
of Maiz Criollo and that of Coker 77 was that the rate of APBR increase 
in Maiz Criollo until day 105 was slower. Also, the decrease in APBR 
at day 95, mainly in the lower internodes, was not as pronounced as 
in Coker 77. 

The lower values of the APBR of the lower internodes of Coker 77 
during part of its linear ear growth phase were not statistically 
different from those of Maiz Criollo, Chapalote, or Nal-Tel. However, 
they may indicate that Coker 77 had a greater sink capacity, which not 
only used photosynthate produced by the canopy, but also utilized 
photosynthate stored in those internodes. 
Partitioning Coefficient 

The partitioning coefficient, or partitioning factor (Duncan et 
al . , 1978), is defined as the division of recent assimilates between 
reproductive growth as opposed to vegetative growth. In a determinant 



71 

crop, particular attention is given to the partitioning that occurs 
just as the final fruit number per plant is determined. This is because, 
before the final seed number is determined, there are not enough seeds 
to utilize all the assimilates potentially available for kernel growth. 
Thus, the plant may continue to grow vegetatively, i.e., produce 
tillers, or store the assimilates. After final kernel number has been 
determined, actively growing kernels develop priority for photosynthates, 
i.e., a polarization of assimilates to the growing kernel established 
in the early phases of embryo development (Loomis, 1945); however, 
actively growing kernels may utilize more assimilate than is produced 
by redistribution or translocation of photosynthates from reserves as 
the season advances or weather condtions are temporarily unfavorable. 

The partitioning coefficient (PC) for the cultivars was calculated 
by the equation PC = (Y.KGR) / (CGRv.AKW) (Duncan et al., 1980). In 
this equation Y represents the final yield, KGR is the rate of dry 
matter increase in individual kernels corrected for energy content and 
energy of formation, CGRv is the rate of total dry matter accumulation 
during the vegetative period also corrected for energy content and 
energy of formation. The CGRv is a measurement of dry matter produc- 
tivity, and in the context of the formula it is equated as an estima- 
tion of net photosynthesis during the period in which kernel number 
was set; AKW is the average kernel weight equated as a measurement of 
the average final capacity of individual kernels to accommodate dry 
matter, i.e., average kernel size (Table 7). 

The PCs of Chapalote, Coker 77, Maiz Criollo, and Nal-Tel were 
0.45, 0.87, 0.75, and 0.99, respectively. This factor is interpreted 
as an estimate of the fraction of photosynthate partitioned to kernel 



72 

growth, as opposed to vegetative growth or storage at the period in 
which potential maximum kernel was set. In fact, PC is. important at 
this moment since it would determine the number of kernels that may 
develop into final yield, provided that climatic and nutritional 
conditions are favorable. 

The lower PC of Chapalote is in agreement with earlier considera- 
tions about distribution of photosynthate between main stalk growth 
and tiller growth. Chapalote was still growing tillers in the middle 
of its linear ear growth phase, presumably, partitioned less of its 
assimilates to ear development. 

The PC is limited by factors other than photosynthate supply, 
since increases in TAC in vegetative components occurred before and 
after anthesis. In other words, cultivars like Chapalote, Maiz Criollo, 
and Nal-Tel, probably had to satisfy the needs of two sinks, the 
reproductive and the vegetative sinks. Thus, the PC limiting kernel 
number also acted to satisfy vegetative needs. However, once vegeta- 
tive needs were satisfied translocation of assimilates from vegetative 
components occurred due to leaf senescence and decrease in leaf photo- 
synthesis, and because one of the components of sink capacity, final 
kernel size, was yet to be satisfied. However, in the period of kernel 
number adjustment, basal kernels may begin to grow sooner than kernels 
at the tip of the ear, simply, because they are receiving photosynthate 
more directly. Thus, an explanation for the lower weight of tip kernels 
may be their shorter filling periods. This may be caused by a late 
beginning of their linear growth rates and an earlier cessation of 
development, probably due to atrophy of vascular bundles as basal 
kernels stop growing. 



73 

Yield Dynamics 

The four cultivars differed significantly in final ear dry weight. 
The ancient races had the lowest yield. The yield of Maiz Criollo was 
34% and 28% higher than that of Chapalote and Nal-Tel, respectively. 
The hybrid, Coker 77, yielded 32% more than Maiz Criollo, and almost 
doubled the yield of the ancient races. The total yield increase of 
Coker 77 was 59% greater than the average yield of the three cultivars 
and 73% greater than the average yield of Chapalote and Nal-Tel. 

The capacity for dry matter accumulation of the cultivars as 
measured by the CGRv did not differ significantly. The statistically 
lower CGRv of Nal-Tel was caused primarily by its lower LAI. Crop 
growth rate is a measurement of the integrated metabolic processes 
that the plant performs for carbon fixation. Among its more important 
components are the photosynthetic and respiratory rates. In this 
experiment a direct measurement of these rates was not done. However, 
the net result of these processes, the CGRv, was not different among 
Chapalote, Coker 77, and Maiz Criollo. This tends to indicate potential 
for similar yields. 

Corn, a determinant plant, ceases vegetative development at 
anthesis or shortly thereafter (Hanway, 1971). As the plant changes 
from the vegetative to the reproductive phase, the PC determines 
allocation of photosynthate for ear development. Chapalote, with the 
smaller PC, continued distributing assimilates to tiller growth. 
The tillers were not only barren and, therefore, represented a 
diversion of growth potential from the ears, but also shaded main 
stalks. Shading, to some extent, could have decreased ear dry matter 



74 
accumulation by reducing photosynthesis of the main ear-bearing stalk. 
This may have caused a poor estimate of the CGRv for the main stalk in 
Chapalote. If the PC could have been computed for the main stalk 
only, it might have been higher. Nal-Tel continued allocation of photo- 
synthate into the stalk component for a longer period than the other 
cultivars (anthesis occurred at day 65 and beginning of linear ear 
growth at day 85), again diverting growth potential from ears. Nal-Tel 
had a high PC, but also had a lower photosynthetic rate (CGRv) which 
did not produce enough assimilate for a high yield. 

Diversion of growth potential from ears is closely shown in Maiz 
Criollo which increased TAC in the stalk component well into its linear 
ear growth phase. This may have resulted because of its lower PC. No 
explanation for the lower PC of Maiz Criollo is evident in the data 
obtained. 

The rate of total day matter increase during the reproductive 
period (CGRr) varied widely among cultivars, and although the CGRr for 
Chapalote and Coker 77 were similar, the CGRr of Coker 77 was maintained 
for a longer period. This fact reflects the greater ability of Coker 
77 to maintain a higher rate of photosynthesis and higher LAI during 
seed filling. The decrease in CGRr in the four cultivars is assumed 
to be caused mainly by a decrease in canopy photosynthesis as the leaves 
aged. This decrease was more pronounced in the ancient races and Maiz 
Criollo than in Coker 77. 

Filling periods, estimated as EEFP or ESFD, did not provide 
explanation for the higher yield of Coker 77. The explanation for the 
higher yield of Coker 77 with respect to Maiz Criollo and Chapalote 
(the cultivars with CGRr not significantly different from Coker 77) 



75 

appeared to be its greater PC that triggered the development of more 
kernels and thus, a greater sink which better utilized photosynthate 
produced and/or translocated. 



'••^ 



SUMMARY AND CONCLUSIONS 

During the 1978 growing season, the hybrid maize Pioneer Brand 
3369A and the inbred Iowa B37 were planted (in a completely randomized 
design with four replications) at populations which provided conditions 
of nearly equal LAI to allow comparisons. This study showed that 
soluble solids were accumulated to a higher degree in the stalk 
component of the inbred line. This suggested that the higher yield of 
the hybrid was caused by its higher PC. 

In 1979 four cultivars were compared in a split-plot arrangement 
with four replications. The cultivars were: two ancient races, Chapalote 
and Nal-Teli a Cuban accession, Maiz Criollo; and a high yielding hybrid 
developed in the south-east USA, Coker 77. Growth analysis indicated 
that the rate of dry matter production in the vegetative phase (CGRv) 
for Chapalote, Coker 77, and Maiz Criollo were not statistically dif- 
ferent. The difference of CGRv between Nal-Tel and the other cultivars 
was probably the result of its lower LAI. 

The similar CGRv of Chapalote, Coker 77, and Maiz Criollo sug- 
gested a similar potential for higher yields. However, the CGRv of 
Chapalote included a heavy growth of tillers and thus only the CGRv 
of the main stalk could be considered potential assimilate for ear 
growth. 

The partitioning coefficient (PC), used as an estimate of the 
distribution of assimilates for ear growth as opposed to vegetative 
growth or storage at the period of kernel number set, was higher in 

76 



77 

Coker 77 than in Chapalote or Maiz Criollo. NaT-Tel had the highest 
PC but also the lower photosynthetic rate (lowest CGRv) which was not 
enough to set a greater sink capacity, thus its low yield. 

Coker 77 maintained a higher LAI and total dry matter accumulation 
rate during reproductive growth (CGRr) than the other cultivars. Also, 
the hybrid had a greater rate of accumulation of ear weight (EGR) than 
Chapalote or Maiz Criollo. 

It follows that Coker 77 had a better combination of sink capacity 
in terms of kernel number and size than Chapalote or Maiz Criollo. 
Chapalote had a relatively high kernel number that did not fully 
compensate for its lower yield, probably because of its lower kernel 
size. Total available carbohydrates (TAC) measured in the stalk, leaf, 
cob, and grain were higher in the hybrid than in the other cultivars. 
High percentage TAC in cobs of Coker 77 suggested a better driving 
mechanism. Translocated TAC from vegetative components contributed 9, 
26, 13, and 21% to the final ear yield in Coker 77, Chapalote, Maiz 
Criollo, and Nal-Tel, respectively. 

The results of this experiment support the conclusion that under 
conditions of equal LAI, high PC, which determines greater sink capacity, 
and a high production of photosynthate during reproductive growth are 
the physiological parameters that cause high yield. 



LITERATURE CITED 

Alberda, T. H. 1962. Actual and potential production of agricultural 
crops. Neth. J. Agric. Sci. 10:325-333. 

Allison, J. C. S. 1964. A comparison between maize and wheat in 

respect to leaf area after flowering and grain growth. J. Agric. 
Sci., Camb. 63:1-4. 

Allison, J. C. S., and P. B. L. Thomas. 1974. Leaf arrangements of 
Rhodesian maize cultivar SR52. Rhod. J. Agric. Res. 12:85-87. 

Barr, A. J., J. H. Goodnight, J. P. Sail, W. H. Blair, and D. M. 

Chilko. 1979. SAS User's Guide, 1979 Edition. SAS Institute, 
Inc., Raleigh, North Carolina. 

Black, J. N. 1963. The interrelationship of solar radiation and leaf 
area index in determining the rate of dry matter production of 
swards of subterranean clover (Teifolium subterraneum L.). Aust. 
J. Agr. Res. 14:20-38. 

Blackman, V. H. 1919. The compound interest law and plant growth. 
Ann. Bot. 33:353-360. 

Bodea, C. 1934. Sugar from corn stalk. Chemic and Industrie 
31:1046-1048. 

Brougham, R. W. 1956. Effect of intensity of defoliation on regrowth 
of pasture. Aust. J. Agr. Res. 7:377-387. 

Brouwer, R. 1962. Distribution of dry matter in the plant. Neth. J. 
Agric. Sci. 10:361-376. 

Campbell, C. M. 1964. Influence of seed formation of corn on accumula- 
tion of vegetative dry matter and stalk strength. Crop Sci. 
4:31-34. 

Chapmann, W. H., E. S. Horner, J. T. Johnson, F. G. Martin, H. A. 

Peacock, and J. M. White. 1978. Hybrid field corn variety tests 
in Central, North, and West Florida. Agronomy Research Report 
AY79-3, IFAS, Univ. of Fla. 

Clark, C. E. 1913. Preliminary report on sugar production from maize. 
USDA, Bur. Plant Ind. , Cir. 111. 

Clarke, J. M. , and J. M. Simpson. 1978. Growth analysis of Brassica 
napus ev. Tower. Can. J. PI. Sci. 58:587-595. 

78 



Crosbie, T. M j j ^q u ^^ 

Daynard, T. B. 1972 Rp7;,i-,-. u- 

moisture percentaar;^nH^"^?'P ^'^°"9 '^lack layer form^t^nn 

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y^'Lln'y-^r^ror'n: ^CroTsc?: 9;?7l:47?^ ''''' ^^^^ -^ ^raln 

'''"B?K£jV^^"'^ -3"l??::?i,^^^J3,/|jJtionships between 

g^am yield of corn. Can. J. Plant'sc]" llzlyV^'/'^^' '"^ '^^ 
Daynard, T R 1 u t 

Dornhoff. g M and r m cu- 23:95-107. 

net P.otos;..e3.s „". f^^,^-- .H.a, .,,3 ,. 

D^-eyer, Johannes. 1980 r^n, .. ' ^ ^^^' 

L.) with different fnn>f ^^^ponse of peanuts (Arachi. h, n 
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Duncan, W. G. 1971 I p;,f = i 

crop Sci. I1..482-4lf '"''"• '-f ^^ea, and canopy photosynthesis. 

Duncan, K. g. 1973 p,.„, ^„ . 

relationships as related ?o'h?;/^"'''^' ""entation and lioht 
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Cambridge Univ' Prp^c^', -^ ^I^OP Physiology, ed I t Fw. 

univ. Press, London and New York dd ?^ In ^^"^• 

Duncan, W. G D B FoT ' 

"^ ' "-"'"v. or r orida 
Duncan, W. G. , A I u.f^i .^ 

yield of corn ' JJ nJi ' ^"^ ^- L. Ragland iQfiq ti, 

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-1,t!-"i-Tpho?os;nth-]i™%f."„V„-^ 



80 

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APPENDIX A 



AMYLOGLUCOSIDASE-INVERTASE PROCEDURE FOR HYDROLYZING 
TOTAL AVAILABLE CARBOHYDRATES (STARCH, SUCROSE) TO REDUCING SUGARS 



1. Weigh 0.1 g of ground, throughly mixed plant material into a 5 ml 
Erlenmeyer flask. 

2. Add 5 ml of distilled water, cap the flasks with marbles and boil 
for 3 to 5 minutes. 

3. To the cooled flask add 5 ml of 0.2 N acetate buffer, and 1 ml of 
enzyme mix. Place flasks in a 44° C water-bath for 10 to 12 hours. 

4. Remove from bath and cool it. 

5. Take aliquots (between 0.2 to 0.5 ml) and put into 15 ml test tubes. 

6. Add deionized water (between 5.5 and 5.8 ml) and shake. 

7. Take aliquots (between 0.2 and 0.5 ml) and put into 15 ml test 
tubes. 

8. Add 1.0 ml of alkaline reagent in each flask, boil for 20 minutes 
and then cool. 

9. Add 1.0 ml of arsenomolybdate reagent in each flask, fill to 10 ml 
with deionized water and mix well. 

10. Read absorbance at 540 nm with a blank solution as zero. Use set 
of glucose standards prepared with same procedure for calibrating 
regression line. 
Acetate Buffer: Mix 3 parts of 0.2 N acetic acid and 2 parts of 

0,2 N sodium acetate. Titrate final buffer solution to pH 4.8 by addition 

of either solution. Add a few crystals of thymol to prevent growth of 

microorganisms. 

87 



88 

Enzyme mix for 50 ml fresh daily: Add 1.25 g of amyloglucosidase, 
1.25 ml invertase, and 3.75 ml of 0.2 N acetate buffer to 45 ml of de- 
ionized water. 

Alkaline reagent: dissolve 25 g of anhydrous sodium carbonate, 
25 g of potassium sodium tartrate, 20 g of sodium bicarbonate, and 
200 g of anhydrous sodium sulfate in 700 ml of deionized water and 
then dilute to one liter. Dissolve 6 g of cupric sulfate in 40 ml of 
deionized water followed by one drop of concentrated sulfuric acid. 
Combine the two solutions. 

Arsenomolybdate reagent: dissolve 25.0 g of ammonium molybdate 
tetrahydrate in 450 ml of deionized water, then add 21 ml of concen- 
trated sulfuric acid. In separate solution dissolve 3.0 g of disodium 
arsenate in 25 ml deionized water. Combine the two solutions. 

Calculate TAC in percentage by: 

<,/jp^Q _ (OP * slope + intercept) * (Dilution factors) ^ -.qq 

wt. (mg) 



APPENDIX B 

TABLES: SOIL FERTILITY ANALYSIS, DRY WEIGHT OF PLANT 
COMPONENTS FOR THE 1978 AND 1979 GROWING SEASONS, TOTAL 
AVAILABLE CARBOHYDRATES, AND PERCENT BRIX READINGS 



Table B-1 . The pH, nitrogen, and double-acid extractable nutrients 
in the soil used in the 1979 experiment. 



Double-acid 
extractable nutrients t 
Soil pH (H2O) N P K Ca Mg 



PPm 

Jonesville fine sandt 6.1 21 116 114 385 60 



to. 05 N HCl + 0.025 N H2SO4 (ratio 1:4). 
=f= Loamy, mixed, thermic Arenic Hapludalf. 



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94 



Table B-5. Average percentage Brix readings for plot for the maize 
cultivars grown in the 1979 growing season. 



Days 
after 








Cult 


ivars 






planting 


Chapalote 


Coker 77 


Maiz Criollo 


NaT -Tel 


55 


3.8 


a* 


3.8 


a 


I 

3.6 


a 


3.8 a 


65 


4.9 


b 


5.1 


ab 


5.0 


ab 


5.5 a 


75 


6.2 


a 


6.3 


a 


5.6 


a 


6.0 a 


85 


8.4 


a 


7.9 


b 


7.6 


b 


8.3 a 


95 


9.0 


a 


8.0 


b 


8.0 


b 


8.4 ab 


105 


8.9 


a 


8.2 


ab 


8.8 


a 


7.2 b 


115 


9.0 


ab 


9.5 


a 


10.0 


a 


7.8 b 


125 


8.8 


ab 


10.2 


a 


8.7 


ab 


7.8 b 



* Means within a row followed by the same letter do not differ 
significantly at the 0.05 level of probability according to 
Duncan's multiple range test. 



95 






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Q. 

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i. 



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to 
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Q (O 1— 

a. 



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CO CM O O 
Lf) LO ID ID 



CO CO O CO 

^ ^ ^ <d- 



CO ro CO <n 

Lf) Lf) LO LO 



n3 (O 
<a- LD 



<0 <T3 re <0 

1— <n <X5 CO 

(£5 LD Lf) to 



re re re re 

1 — CO LD CVJ 

o^ 00 CO cr> 



re re re re 

WD 1— <Tl 00 
VD «J3 LD ys 



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cn 00 00 <Ti 



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CTl VO I— CT> 

to vD in VD 



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r— CM r— 1— 

cn 00 00 cri 



re re re re 
to 00 o CM 
to to to to 



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LD 1 — to in 
00 00 r-^ 00 



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CO CO CO CO 



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LO to LO LD 



* 

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CO CO CO 


CO 


^ ^ ^ ■53- 


to to to ^ 



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3: o o h- 
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to to CM CO 

cn 00 cn cn 



re re re re 

LO I— to CM 

cn 00 CO en 



re re re re 
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00 r-» r-~ 00 



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r-«r^toi^ r--tDtor^ 



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to 



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to 



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cn 



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en r-- 


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en <T> cri CO 



to to to to 
CO (Ti <n r^ 

<Ti CO <T) r-- 



(O (O (^ 

en t— <a- 
c o n- 



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en 



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I— o 



to to to to to to to 

lO r— CO «d- CM CO r~ 

CTl O 1 — <Tl O r— O 



(0(Q(0lT3 fO(^(0(0 to to to to 

coor^<^ OOLOCTi Lr>r»-kOLn 
ocncnr^ enoocc (nocncD 



to to to to to to to to to to to to 

cnooco<d- COOOCOUO >;3-0«a-<JD 
c6o6cdr<! oocnenco cooooeo 



to to to to to to to to to to to to 

cvjr--.r-^co cvjcnocM ir> '^ id i — 
oor-»r~-u3 cooo<Tir- cocrir~.t^ 





^ XI 




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to 





to to to to to to to to 
(Til— voco r— r-.r-»co 
t--.cor>~in cocovoio 



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in 



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to 
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H- 


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3 != 


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r- Q 




r— 


^— 


o o 


o 


M- +J 






s- 


C CD 


o 


o c 




•^ 'r- 


N 


-M T3 




IT3 S- 


to 


C O 


s: 


•1- O 




J3 O 


II 


E (O 




o 


o 


CJ >> 


s: 


■(-' 




0) -r- 




-tJ r— 


• M 


(O •■- 


r-. 


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t^ 


1 n3 




0) J3 


s_ 


-a o 


<u 


o S- 


^ 


C Q. 


o 


s- 


CJ 


Qj ^- 




+J o 


II 


c 




■r* ^— 


o 


<u 


(_) 


c > 




(O <D 




^— 


• A 


c 


<u 


•I— in 


4J 


jc o 


o 


4J • 




•r- O 


to 


3 


a. 


OJ 


to 


V) x: 


-C 


c +-> 


o 


to 




OJ +J 


II 


s: (0 





BIOGRAPHICAL SKETCH 

Raul Rene Valle Melendez was born 18 March 1951 to Raul Rene and 
Georgina Melendez de Valle in Tegucigalpa, Honduras. 

He attended primary and secondary schools in Honduras finishing 
in 1968. In 1969 he enrolled at the Universidade Federal de Rio de 
Janeiro, from which in 1971 he transferred to the Faculdade de Agronomia 
e Zootecnia "Manoel Carlos Goncalves" in Pinhal, Sao Paulo, Brasil, 
seeking a specialization in phytology. He received an Engenheiro 
Agronomo degree in 1972. 

After graduation, he worked with the Secretary of Natural Resources 
in Honduras as assistant and then chief of a seed processing facility. 
In 1974 he specialized in crop production at the International Center 
of Tropical Agriculture (CIAT) receiving first place among 25 agronomists 
of various nationalities. In 1975 he was appointed Sub-Director of an 
Agricultural Region of Honduras (Region Agricola Centro-Oriental , 
Danli, El Paraiso), and in 1976 was transferred to Catacamas, Olancho, 
to work as Director of "Raul Rene Valle" Experimental Sub-Station. In 
January 1977, he entered the Graduate Program at the University of 
Florida and received a Master of Science degree in December 1978. He 
began studies for the Doctor of Philosophy degree at the University of 
Florida in January 1979. Upon receiving the Ph.D., he expects to return 
to his country and contribute to its agricultural development. Raul Rene 
Valle Melendez speaks fluently Portuguese, English, and Spanish (mother 
tongue). He is a member of the American Society of Agronomy, the Crop 
Science Society of America, and the Soil Science Society of America. 

97 



I certify that I have read this study and that in my opinion it 
conforms to acceptable standards of scholarly presentation and is fully 
adequate, in scope and quality, as a dissertation for the degree of 
Doctor of Philosophy. 




F. P. Gardner, Chairman 
Professor of Agronomy 



I certify that I have read this study and that in my opinion it 
conforms to acceptable standards of scholarly presentation and is fully 
adequate, in scope and quality, as a dissertation for the degree of 
Doctor of Philosophy. 



^. -yws. Q2*Jl 



D. E. McCloud 
Professor of Agronomy 



I certify that I have read this study and that in my opinion it 
conforms to acceptable standards of scholarly presentation and is fully 
adequate, in scope and quality, as a dissertation for the degree of 
Doctor of Philosophy. 



W. G. Blue 

Professor of Soil Science 



I certify that I have read this study and that in my opinion it 
conforms to acceptable standards of scholarly presentation and is fully 
adequate, in scope and quality, as a dissertation for the degree of 
Doctor of Philosophy. 




K. {^. Boote 

Associate Professor of Agronomy 



I certify that I have read this study and that in my opinion it 
conforms to acceptable standards of scholarly presentation and is 
fully adequate, in scope and quality, as a dissertation for the degree 
of Doctor of Philosophy. 



miu^ 



af/^^ 



W. G. Duncan 
Professor of Agronomy 



I certify that I have read this study and that in my opinion it 
conforms to acceptable standards of scholarly presentation and is 
fully adequate, in scope and quality, as a dissertation for the degree 
of Doctor of Philosophy. - 



'^I '~7h 



I 



V-^K^T^-^^ 



E. S. Horner 
Professor of Agronomy 



This dissertation was submitted to the Graduate Faculty of the College 
of Agriculture and to the Graduate Council, and was accepted as partial 
fulfillment of the requirements for the degree of Doctor of Philosophy. 



March 1981 







'College of Agr><?01ture 



Dean for Graduate Studies and 
Research 



UNIVERSITY OF FLORIDA 



3 1262 08554 8484